The Costs and Benefits of Six-Man Rotations

Planning a starting rotation for 2021 carries innumerable pitfalls. Nearly every pitcher in the league saw a reduced workload last year, and they did it in strange circumstances to boot. It’s not merely that the short season set everyone’s innings back — though that’s a huge component. A large number of cancelations and postponements also meant more doubleheaders and more cobbled-together games, another way to throw pitchers off their rhythm.

Put it all together, and protecting arms sounds like an appealing plan for 2021. The Mariners announced that they’ll use a six-man rotation next year, a continuation of the plan they leaned on for all of 2020. The Red Sox are talking workload management. Since initially publishing this piece, Jeff Zimmerman pointed out that the Tigers will use a six-man rotation as well. Is an embiggened rotation the solution to this universal problem? Let’s do the math.

It depends, first of all, on what you give up. The innings tradeoff of a six-man rotation is straightforward. Giving your pitchers an extra day off limits their workloads, naturally enough. The math on that is straightforward if you assume it doesn’t affect their in-game workload. Take a pitcher who averages six innings per start. In a five-man rotation, that’s 192 innings of work. Adding a sixth pitcher to the rotation cuts that down to 162 innings.

How much do those 30 innings of work matter when it comes to health? I’ll level with you — I’m not sure. We simply don’t have the data to say with any amount of certainty, because the number of comparable situations is so small. Pitchers have light workloads all the time, but in most cases it’s due to age or injury. Looking at what a 21-year-old pitcher did in first building up stamina probably can’t tell us much about how many innings Jake Odorizzi, to pick a random example, should throw in 2021. Likewise, a pitcher’s workload in his first year back from Tommy John surgery can’t tell us how many innings Trevor Bauer can be effective for.

We do have a few points that we could look at for a natural experiment. The ‘94 strike depressed every pitcher’s workload, injured or not. The problem, here, is that due to the length of the pre-strike season, no healthy pitchers added innings in the way we expect to see this year. Heck, Greg Maddux threw 202 innings in 1994. He threw 209.2 the next year. Lance Lynn led baseball with 84 innings pitched last year, and we project him for 191 in 2021. The increase in workload isn’t comparable.

The same problem exists for the 1981 season. To make matters worse, pitcher workloads barely resembled today. In 1982, 50 pitchers threw 200 or more innings. In 2019, only 15 hit that threshold. Speculating on what will happen to modern pitchers based on what happened to Charlie Hough in the 80s hardly seems like a productive approach.

Fine, then: we won’t assume any benefit, at least from a performance perspective, from adding an extra pitcher to the mix. That doesn’t mean there’s no benefit at all, however. An extra day of rest gives pitchers more time to recover between starts. Assuming no benefit from that rest doesn’t make much sense to me.

To approximate this effect, I looked at the number of days of rest between each start made in baseball this year. I threw out starts made on three days’ rest (there were only 14) and starts made on seven or more days of rest — those aren’t rotation- or schedule-based. That left me with 1,291 starts to look at.

You might think that the right thing to do is simply look at league ERA on four, five, and six days’ rest. You’d be wrong, though, due to population effects. Pitching on standard rest — or getting an extra day to recover — isn’t a decision that teams make at random. They have motives, and those motives depend on the identity of the pitcher. Getting an ace an extra turn makes sense. Using a bad pitcher as infrequently as possible makes sense. If you ignore this distributional effect, you’re liable to end up with a confusing answer.

Instead, I focused on how each pitcher fared relative to their seasonal line. I focused on xFIP, since we’re looking at noisy individual game lines, and compared starts on each relevant amount of rest to a pitcher’s overall seasonal line. In other words, if a pitcher had a 4.50 xFIP overall, a 4.60 xFIP on regular rest, and a 4.40 xFIP on five days’ rest, we’d say that four days’ rest increases his xFIP by 0.1 while five days’ rest decreases it by 0.1, an overall difference of 0.2 runs.

The numbers don’t need to be exactly symmetrical like that example, of course. Pitchers don’t make an equal number of starts with each type of rest, and they also don’t pitch an equal number of innings per start in each case. The overall point, however, is that extra days of rest seem to have a small but beneficial effect on pitchers’ results. For each extra day of rest, their xFIP improves, relative to their seasonal average, by roughly 0.06.

I won’t claim that this effect is obvious and incontrovertible. It’s tiny, below the threshold of statistical significance. Break it down into strikeouts and walks, and it’s even smaller; an extra day of rest is associated with a 0.3 percentage point increase in strikeout rate but also a 0.15 percentage point increase in walk rate.

In other words, the extra day of rest doesn’t appear to be worth that much. If you take my math at face value and assume that all previous starts were on regular rest — a poor and lazy assumption that I’m absolutely going to make for the sake of time — that works out to something like 6.5 runs over the entire season. Though I’m tired of saying it, I still will — I have very little confidence in this number. As a ballpark guess, however — and pun very much intended, I have a reputation to uphold — I think it gets the job done.

Okay, so teams can get an indeterminate injury benefit and also maybe save 6.5 runs by going to a six-man rotation. Let’s talk cost. The cost is clear: your top five starters all throw roughly 30 fewer innings, and those innings get replaced by your sixth-best starter. Sorry, Max Scherzer, but it’s Austin Voth’s turn to shine, and so on.

The cost of this downgrade varies by team. It depends on two things: the average quality of your existing rotation and the quality of your sixth starter. To approximate this cost, I took Depth Charts projections for every team in baseball and then calculated the existing rotation’s average projected ERA and the sixth-best starter’s projected ERA. The difference between those two numbers, over the 150 innings we’re granting our sixth starter in this scenario, will be each team’s cost of going to six starters.

Take the Royals, for example. Their best five starters — Brady Singer, Mike Minor, Kris Bubic, Brad Keller, and Danny Duffy — project for an average 4.70 ERA. Their prospective sixth starter, Jakob Junis, projects for a 4.87 ERA. That difference, expressed over 150 innings, is worth roughly three runs. Per my admittedly hazy math, the Royals might be better off with a six-man rotation even before considering the injury benefits.

With the caveat that all of this is ridiculous fantasy-land math that I did on the back of a not-particularly-large envelope, here’s how many more runs each team should expect to allow if they go to a six-man rotation next year:

Sixth Starter Costs by Team
Team ERA Gap Extra Runs
Royals 0.17 2.9
Orioles 0.21 3.5
Angels 0.24 4.1
Giants 0.27 4.4
Rangers 0.29 4.8
Twins 0.34 5.7
Athletics 0.35 5.8
Phillies 0.35 5.8
Braves 0.37 6.1
Cardinals 0.38 6.4
Cleveland 0.41 6.8
Marlins 0.43 7.1
Tigers 0.43 7.1
Pirates 0.43 7.1
Brewers 0.43 7.2
Cubs 0.43 7.2
Blue Jays 0.48 8.0
Rays 0.49 8.2
Dodgers 0.50 8.4
Mariners 0.51 8.5
Reds 0.52 8.7
Diamondbacks 0.59 9.8
Mets 0.62 10.3
Red Sox 0.66 11.0
White Sox 0.66 11.0
Yankees 0.73 12.2
Astros 0.75 12.4
Nationals 0.78 12.9
Padres 0.91 15.1

You might notice that the Rockies don’t appear on this list. That’s because if you asked me to project their sixth starter, I’d mutter “Hey what’s that behind you?” then run away when you turned to look. They obviously have someone detailed for that job, but I don’t find assigning it to any specific player particularly meaningful.

Do you want a solid takeaway from this article? Tough luck. Some of these teams could easily shift to using a sixth starter. Some would need to give up a little more. In no case is it a massive effect, depending on how you define the word “massive” — even the top-heavy Padres would only cost themselves 15 runs (roughly 1.5 wins) by adding a sixth starter, and that’s if there’s no benefit from an extra day of rest.

Will more teams shift to six-man rotations? I think so. It’s prudent — pitcher health is tenuous in the best of times, and while I’m certainly no biomechanical expert, extra rest meaning extra health hardly seems farfetched. It might help, performance-wise — teams probably have better data on this than I do. It’s cheap, runs-wise — thinning out the rotation is a matter of single-digit runs over an entire season for most teams. Weigh those three factors, and I don’t think Seattle will be the only team using this plan.

Update: This article has been updated to reflect the fact that the Tigers plan to start 2021 with a six-man rotation at least part-time.





Ben is a contributor to FanGraphs. A lifelong Cardinals fan, he got his start writing for Viva El Birdos. He can be found on Twitter @_Ben_Clemens.

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If a starting pitchers need a 6-man rotation (thus pitching once per week) to stay healthy then I would postulate that starting pitching (throwing ~100 pitches) is not a healthy and sustainable activity. Instead, baseball should move to a pitcher-by-committee approach aka bullpen games (ignoring entertainment value for a second)

The only other sport where someone needs a full week of rest to recuperate is football, whereas baseball, basketball, and hockey all play more frequently and generally all the players play each game.

My point is thus: if you need a week to recover from a physical activity, you should probably stop doing that activity rather than just getting more rest.

SirLancelittle
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SirLancelittle

I think baseball has, in fact, demonstrated that “starting pitching is not a healthy and sustainable activity”. 25% of active MLB pitchers have had Tommy John surgery (and that isn’t double counting people who’ve had it twice, counting other pitching injuries, or counting those that never recovered and were forced to retire).

Every pitcher is one bad day away from missing months or years to injury.

shoewizard
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shoewizard

I did not realize the percentage was that high. I believe you, but would like to know more. Can you point in the direction of this data ?

SirLancelittle
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SirLancelittle

Honestly, I don’t know who gathered the data; that number is just quoted everywhere. Google “percentage of mlb pitchers with tommy john” and you’ll find dozens of article with similar numbers.

An example from a reliable source: https://www.billjamesonline.com/tommy_john_surgery_in_major_league_baseball/

tung_twista
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tung_twista

It goes deeper than that.
Professional sports in general is increasingly becoming a less healthy and sustainable activity as people push limits of human body.
Not saying professional athletes are not healthy, because they clearly are.
And teams are paying more attention to minimize injury, which is a good thing.
But fundamentally, the incentive to risk increased chance of injury for better short-term performance is too strong when millions of dollars are at stake.

Dylan Feldman
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Dylan Feldman

“I would postulate that starting pitching (throwing ~100 pitches) is not a healthy and sustainable activity.”

It’s not. And we know it’s not. It never has been. Guys who have long careers of sustained success are, generally speaking the exception.

However, there’s still a pure numbers problem that makes going to full bullpen games impossible. The average team pitches ~1450 innings to pitch in a given season. Even if you factor in 15 pitchers, you’re still having guys throwing 90+ innings per year. And that’s with the additional strain of pitching on a greater number of days if you’re going to bullpen games. Which means more warming up, fewer rest days, etc. You would probably need to find a way to rotate through 20 pitchers to make that work, which also means finding 20 MLB quality pitchers and then having enough roster flexibility to do so.

There’s no form of pitching rotation that is really a sustainable and healthy activity. But I don’t see bullpen games being any better in that regard.

RonnieDobbs
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RonnieDobbs

I am sure bullpens are worse. Half the guys in the pen are there because they can’t handle much of a workload – combine that with a 100% mentality and you realize why SP are important – they are the best of the best.

yaro
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yaro

Need is not the question. The question is what’s optimal from a health and performance stand point. Pitchers don’t just pitch 2 games X days apart. They go through a season where varying degrees of injury are basically constant. Also the comparison to other sports is moot as pitching is literally a recipe for injury. It’s a very precise repeated task done at a very high intensity. Thats literally a formula for injury.

RonnieDobbs
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RonnieDobbs

Pitchers would not magically stay healthy with a sixth starter or any other magical idea. There is 100 years of history that proves that it is OK to throw 100 pitches and throw every fifth day. Nobody ever does this, but look at the actual humans in the big, bad bullpens. They have short windows of effectiveness and extreme injuries. There is no real data to support the theories that this can be done better. People will create models and alter then to make them to support their ideas, but there is not real history of more effective pitching staffs or ways to keep people healthy. I am certain that 5 days is an adequate ammount of rest – history has proven so. How many times have you heard a SP complain that 5 wasn’t enough. They should be the ones we are asking. Power lifters rest a week between sessions – there is precedent for long rest periods but it should have intent… much as the 5 day starter routine does. This isn’t a real problem as much as an excuse for teams not attempting to build legitimate staffs.

tomerafan
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tomerafan

Pitchers would not magically stay healthy with a sixth starter or any other magical idea.

Except in Japan. Although, admittedly, 5-man versus 6-man rotations is not an isolated variable in that case.

There is 100 years of history that proves that it is OK to throw 100 pitches and throw every fifth day.

There is not, however, 100 years of history that pitchers can throw at the velocity and torque that the modern game presents, and arm injuries have increased over time. Again, not a perfect variable set because we also know that many old arm injuries weren’t reported, and pitchers just sometimes washed out rather than have surgery. But, still, comparing pitchers 100 years ago to today when we’re talking about whether the human arm can survive the pitching motion is not even in the realm of an apples-to-apples comparison.

Nobody ever does this, but look at the actual humans in the big, bad bullpens. They have short windows of effectiveness and extreme injuries. There is no real data to support the theories that this can be done better.

You’re making a different argument now. There’s clearly a different set of variables affecting bullpen usage including up-and-down during a game, days’ usage, etc. Your point is getting farther away from 5-starters versus 6-starters.

People will create models and alter then to make them to support their ideas, but there is not real history of more effective pitching staffs or ways to keep people healthy.

Except, again, Japan.

I am certain that 5 days is an adequate ammount of rest – history has proven so.

Your definition of certain must be different from mine. And history hasn’t proven what rest is adequate for the velocity, torque, arm action, etc. we’re seeing today. It would be absurd to say that since seat belts don’t meaningfully change fatality rates in collisions at 20 mph, they’re not needed for safety on the Interstate.

How many times have you heard a SP complain that 5 wasn’t enough. They should be the ones we are asking.

Virtually every significant pitcher who has come over from Japan has made this point, and they are the ones with a comparison. American pitchers don’t know differently. There was a rumor – when Darvish complained and then quickly got quiet – that the MLBPA didn’t want pitchers talking about 6-man rotations because it would, in theory, decrease the purported value of each starter by depressing their stats other than on a rate basis. Completely unsubstantiated, as far as I know. It’s reasonable to understand, though, that most American pitchers who’ve only thrown every 5 days would want to stick with that routine, with all we know about athletes being creatures of habit.

Power lifters rest a week between sessions – there is precedent for long rest periods but it should have intent… much as the 5 day starter routine does.

Shoot, I have to rest up now when I coach my kid’s youth baseball team, because I’m older than I used to be. But I’m not more a good comparison to Noah Syndergaard than a power lifter is. The human arm is not designed to throw an object at that velocity and with that kind of torque, and as a result it breaks sometimes.

This isn’t a real problem as much as an excuse for teams not attempting to build legitimate staffs.

I assure you, arm injuries are a real problem. Players have a lot invested in their own careers, and owners often have a lot of money invested in the guaranteed contracts they hand out to established pitchers. Why shouldn’t both sides be looking to all avenues of ensuring that pitchers are durable for longer?

bosoxforlife
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bosoxforlife

It is much more than just not getting hurt! It is all about maximizing value. How do teams get the most innings out of their best pitchers without sending them to Dr. Andrews? The fact that 6-man rotations are even being contemplated proves that nobody has figured out the equation, even after 145 years of organized baseball. The idea of limiting Jacob deGrom to 150 innings is ridiculous if those innings in which he isn’t where he should be are filled by Paul Sewage or Robert Gopherball. How does that help the Mets win? I can understand paying a stud $25-30 million but only if the the program includes a minimum of 200 innings. The way things are headed we may have seen the last qualifier for the ERA crown unless some bottom feeder keeps sending out some stiff to eat innings and he wins the award with a 6.27 ERA.

bosoxforlife
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bosoxforlife

The answer is simple. Some can and some can’t and nobody has ever been able to predict if or when it is or isn’t going to happen.