A Potential Fix for Patrick Corbin

At the trade deadline, the Nationals cleaned house. Max Scherzer and Trea Turner went to the Dodgers, Brad Hand to the Blue Jays. Kyle Schwarber? He’s now on the Red Sox. Daniel Hudson? A Padre. Josh Harrison and Yan Gomes are Athletics, and for some reason, the Cardinals decided to acquire Jon Lester. What’s left of the Washington Nationals are bits and pieces from their 2019 World Series run, some young prospects, franchise mainstay Ryan Zimmerman, and Juan Soto.

It’s probably too soon to ask when the Nationals will find themselves contending again. However, both their retention of Soto and their acquisition of major league-ready prospects Josiah Gray and Keibert Ruiz potentially indicates that the front office at least hopes the team will play meaningful baseball sometime in the near future. While things may look a bit hazy in the immediate wake of the deadline, one player whose performance could influence the timing of the Nationals next competitive window is Patrick Corbin, who still has three years left on his contract after 2021.

Over the last two seasons, Corbin hasn’t been especially good. After a sparkling debut season with Washington, one that culminated in the aforementioned championship, Corbin has been average or worse since. He pitched 202 innings in 2019 to the tune of a 3.25 ERA, a 28.5% strikeout rate, and a 8.4% walk rate. He was worth 4.7 WAR, a figure that ranked 14th in the majors (it was good for third on his own team). The Nationals assembled a super-staff that year, and as I asserted on this very website, the pickup of Corbin the prior offseason made a huge difference in the team’s success not only in the regular season, but in the postseason as well.

Corbin was a fireman in the 2019 playoffs, doing whatever the Nationals asked of him. He threw 23.1 additional innings that October, serving as both a starter and a high-leverage, multi-inning relief arm. After Scherzer could go only five innings in Game 7 of the World Series, it was Corbin who came in on three days rest and shut down the Astros for three innings, allowing Washington to take a seventh inning lead and never look back. That postseason asked a lot of Corbin, and at least according to Zimmerman, that might be a reason why he’s been so unlike himself since.

But I don’t buy it. Yes, the top line numbers have been troubling. In 2020, Corbin pitched to a 4.66 ERA and a 4.17 FIP. This year, he’s up to a 5.83 ERA and a 5.52 FIP. He still struck out 20.3% of batters in 2020, but that’s down to just 17.9% this year, after posting a near-30% strikeout rate from 2018-19. This year, balls are leaving the yard at a rate he hasn’t seen before; no other pitcher has allowed more runs, earned or not, than he has. At 32, this shouldn’t be the end of the line for Corbin, and I don’t think it is. But he’s clearly not the same pitcher who posted 13.7 WAR over a three-year stretch spanning from 2017-19, a figure that put him inside the top-10 in baseball.

I don’t think overuse explains Corbin’s issues, though. While he did experience a small velocity dip last season, with his average four-seamer dropping from 91.8 mph to just 90.2, it has ticked back up in 2021, to 91.9 mph. He’s actually throwing his sinker, an offering Corbin uses more often than his four-seamer, 2 mph harder than he did last year and as hard as he did back in 2017. I think overuse would likely show up in the velocity, and while Corbin’s never been a big velocity guy, it doesn’t appear to be an issue, at least in comparison to his past data.

What is puzzling, though, are the results Corbin is getting on his best pitch: the slider. In 2019, Statcast data estimated the pitch as being worth 26.8 (!) runs above-average, making it the fourth-most valuable pitch in the game. If Corbin’s slider isn’t working — as it hasn’t been the last two years — he’s not himself. It’s still a good pitch, but there’s an enormous difference between getting a 51% whiff rate on the offering, as he did in 2019, and a 38% whiff rate on it, as he’s done the last two years. “Fixing” Corbin’s slider would go a long way in fixing him, so that is where he must start.

And I think the fix is pretty simple: Corbin is generating too much extension. Corbin’s extension has ticked up big time on all of his pitches and it may be a prime reason for his ineffectiveness. In 2019, Corbin was releasing the ball 6.2 feet from the rubber on average. (I wrote more about extension in a piece on effective velocity and Bailey Falter.) In 2020, that average jumped to 6.7 feet. In 2021, it’s gone up even more, to 6.8 feet on average. His extension has jumped on all of his pitches, but let’s take a quick look at how he’s released his slider over the last five seasons:

Corbin’s slider extension jumped by 0.8 feet from 2019 to ’21. This means that when Corbin throws the pitch, he’s now doing so 9.6 inches closer to home plate than he was two years ago. Why does this matter? Well, for a pitcher like Corbin, who has a big, looping, curveball-ish slider, it means he’s missing out on a ton of gravity that could help him miss bats.

This is what the slider looked like in 2019:

And here it is now:

From those two pitches, it’s not easy to see the difference. They’re hand-selected, and since the camera angle is from center field, there’s almost no way to see the change in extension. But does extension really matter? And is 10 inches really all that meaningful? At least for Corbin, the answer to both questions is yes.

Because Corbin’s biggest issue is a lack of whiffs, rather than a jump in opponents’ quality of contact, I focused on swinging strike rate. Does extension influence Corbin’s swinging strike rate on the slider? To answer that question, I scraped every one of Corbin’s sliders in the Statcast era (2015-21) and grouped them into buckets by release extension. I filtered out the buckets that had fewer than 100 pitches in them, and was pretty astounded by the results. Extension is pretty well associated with swinging strike rate:

By including 2020 and ’21, though, you could argue that this may not be as much of a causal relationship. Could the decrease in swinging strike rate be the result of some other confounding variable that just happens to look like it correlates to an increase in extension because Corbin has upped his extension this year? In other words, is some other factor leading to the decrease in slider effectiveness, one that means the increase in extension doesn’t have much or anything to do with it? To answer this question, I removed 2020 and ’21 from the data and conducted the exact same process. As we see, on pitches where Corbin extended further, hitters still swung-and-missed less often:

You’ll notice two things from this plot: the relationship is probably still there, but there’s also a missing group. Why is there no 6.7-6.9 feet release extension group? That’s because prior to 2020, Corbin didn’t throw enough pitches in that group to qualify for my 100-pitch minimum. Even in a comparison from 2019 to 2020-21, you can see just how drastic Corbin’s change in extension really is by looking at the total distribution of his sliders:

And this matters for more than just swinging strike rate. Corbin’s slider is way more effective when it’s thrown with less extension. Look at the pitch’s run value per 100 pitches, filtered into those extension groups. For pitchers, lower is better:

His slider is excellent when thrown at 6.3 feet of extension or less, generating at least 1.5 runs above-average per 100 pitches in any of those groups. As his extension trickles up, though, the pitch becomes much worse, even being slightly below-average when Corbin throws it with at least 6.7 feet of extension. And, remember, his average extension on the pitch this year is 6.8 feet. That’s a big problem. And this relationship holds pretty well even when looking at pre-2020 data:

To say the last couple of years have been underwhelming for Patrick Corbin would be an understatement. Over a three-year stretch, the lefty was one of the 10 most valuable pitchers in the game; this season, he’s allowed more runs than any other hurler. Luckily for him, though, I don’t think this decline in performance is permanent, or is even tied to his high usage during the 2019 postseason. An increase in extension has led to slightly less vertical movement on the breaking ball, which may be just enough for hitters to make contact and avoid a whiff. If Corbin’s slider isn’t working, his overall effectiveness isn’t there. But if he looks back at his mechanics and makes a meaningful change to his extension toward home plate, he may be able to rediscover the 50% whiff rate on his slider that was once a staple of his game.