The Mariners Add Sergio Romo to Shore Up Their Fun Differential

I have a schematic in my head for how a major league team can assemble a dominant bullpen. You, as someone who reads FanGraphs, probably have an idea in your head for how a major league team can assemble a dominant bullpen. They’re probably the same ideas – assemble a stable of guys who can throw 98. If that doesn’t work, assemble a stable of guys who can throw 99, and add wipeout sliders until it clicks.

The Mariners had one of the best bullpens in baseball last year, and they nearly rode that unit – and their resulting excellent performance in close games – to the playoffs. You don’t coin “fun differential” if you don’t have a good bullpen. Yesterday, the team bolstered this year’s version by signing Sergio Romo to a one-year deal worth $2 million (or up to $2.25 million with incentives). In doing so, they added to a truly interesting unit that will look to back up last year’s spectacular performance while eschewing the way that their competitors look to combine relievers, at least to a degree.

In 2021, 135 relievers threw at least 20 innings while averaging 95 mph or higher on their fastballs. The Mariners will employ one of them – Diego Castillo – this year, and he’s only on the list due to his rarely-used four-seamer, as his sinker dipped below 95 last year. They have some other flamethrowers in their ‘pen – Andrés Muñoz throws 100, but missed most of 2021 with injury. Ken Giles might qualify as a flamethrower eventually, but his recovery from Tommy John so far has him topping out at 95.

That’s not to say Seattle has no one who throws hard. Drew Steckenrider, who turned in 67.2 superlative innings of relief last year, sits 93-95 mph. Anthony Misiewicz lives in the same range velocity-wise, though his results haven’t matched up. Erik Swanson’s velocity has been all over the place, but he might qualify as well in a good year.

In signing Romo, though, the Mariners are adding to a strategy that proved effective last year. Steckenrider was one of their best relievers, but he didn’t lead the bullpen in WAR. That honor fell to Paul Sewald, who struck out nearly 40% of the batters he faced while sitting in the low 90s. Casey Sadler, who will unfortunately miss this season due to injury, did yeoman’s work for the bullpen using his 93 mph sinker. That trio kept the bullpen afloat even after the team traded Kendall Graveman, their best reliever of the first half, at the deadline.

Romo isn’t a lower-90s guy; he’s a lower-80s guy. He’s sui generis; at 39, he has 14 years of near-identical-looking relief work under his belt, and good work at that. His career ERA is a sterling 3.10, though that’s exaggerated by his time pitching in San Francisco. Even after controlling for that advantage, his career ERA- is 79; he’s been 21% better than league average at preventing runs.

He’s done it almost exclusively with guile. He’s thrown 4,055 fastballs in his career, and the fastest clocked in at 92.4 mph (in 2009). He hasn’t touched 90 since 2015. His fastball hardly turns heads – and yet he’s recorded an above-average swinging strike rate in every year of his career. How? His slider has mowed down the opposition, year after year, regardless of how hittable his fastball might look.

How, then, did the Mariners snag Romo for only $2 million? His 2021 season raised questions about whether this might be the end of the road. He posted the lowest strikeout rate of his career and the second-highest walk rate. In the second half, he got shelled; he had a 5.46 FIP, a 5.16 ERA, and allowed a scary .337 wOBA to opposing hitters. By season’s end, the A’s had mostly stopped using him when they had the lead; he didn’t enter a single game where the team was tied or ahead in the last two weeks of the season after blowing a save on September 19, and he’d already been ceding high-leverage work before then.

So are the Mariners getting a lemon? Perhaps – but Romo has recovered from similar swoons before. He suffered the same end-of-season lumps in 2020 before recovering to start the ’21 season. A month of scuffling performance doesn’t mean that Romo’s tried-and-true pitching style is suddenly cooked. If you think there’s a 50% chance that Romo is on the way out, you can look at the Mariners signing as paying him like a $4 million reliever if he’s right; a 50% chance of zero value and 50% chance of $4 million in value works out to his $2 million salary.

Is it over-fitting a narrative to cast the Seattle bullpen as the guys who throw slow when they don’t have anyone remotely like Romo? Quite possibly. Romo and Sewald aren’t even close to being the same pitcher. Having average or near-average velocity isn’t the same disadvantage as throwing BP fastballs. And velocity isn’t even a perfect metric for describing how relievers operate; Sewald’s best pitch is his flat, rising fastball, which generates swinging strikes galore via shape rather than speed.

Instead, count the Romo signing as doubling down on the idea that the team can create a valuable bullpen that doesn’t look like a valuable bullpen. Last year, we projected them as the fourth-worst reliever group in baseball, and were wrong by quite a bit. This year, our Depth Charts have them 20th in reliever WAR; again, our projections don’t see how the Mariners will assemble a dominant relief corps.

Last year’s iteration of the Seattle ‘pen didn’t fluke their way into success, at least in the way the baseball analysis community tends to define flukes: they posted excellent strikeout and walk numbers, missed a ton of bats, and limited loud contact while generating grounders and pop ups. Many of these relievers had career years in those underlying statistics, though; their top five relievers all had career-best WAR marks.

Will they do it again? I’m not here to give you that answer. How the bullpen performs won’t necessarily be the difference between making and missing the playoffs; how the team’s bumper crop of young hitters and starters develop will likely be far more consequential. But I love the symbolism of it: the Mariners, the team with a bullpen that succeeds for reasons no one quite saw coming, are adding a pitcher who succeeds for reasons that no one can quite pin down even now.