2021 Review — Starting Pitcher xK% Overperformers

Yesterday, I shared my latest pitcher xK% equation and identified and discussed the nine starting pitchers that most underperformed those marks.

xK% Overperformers
Name Pit/PA L/Str S/Str F/Str 3-0% K% xK% Diff
Jacob deGrom 3.78 20.2% 33.3% 27.1% 2.5% 45.1% 40.6% 4.5%
Shane Baz 4.04 22.1% 27.5% 29.0% 6.1% 36.7% 32.7% 4.0%
Nestor Cortes 4.07 25.3% 17.7% 32.3% 5.6% 27.5% 24.7% 2.8%
Lance Lynn 4.06 21.4% 20.6% 32.9% 4.0% 27.5% 24.9% 2.6%
Michael Kopech 4.05 28.9% 23.3% 27.0% 5.3% 36.1% 33.6% 2.5%
Stephen Strasburg 3.81 34.5% 19.0% 17.0% 11.6% 22.1% 20.0% 2.1%
Carlos Rodon 4.16 23.3% 25.4% 30.4% 4.1% 34.6% 32.5% 2.1%
Gerrit Cole 4.09 26.5% 23.4% 27.8% 2.3% 33.5% 31.4% 2.1%
Max Scherzer 4.04 23.6% 26.2% 28.1% 3.7% 34.1% 32.1% 2.0%
League Avg 3.90 25.7% 19.3% 28.1% 4.6%

Ignore the fact that Jacob deGrom tops our list and instead marvel at his 40.6% xK%! Like….reaaaaalllly? deGrom recorded the most number of innings with a 40%+ xK% on my file, which goes back to 2015. That’s insane. Oh, and he’s the only starter in the group of 21 pitcher seasons with a 40%+ xK%. That 33.3% S/Str is just absurd. Here’s to hoping he manages to return healthy with no performance effects, not just for my LABR team, but to cement his auto-HOF induction. I worry that his current 1,261 innings might not be enough just yet, despite the high quality of those innings.

Shane Baz, our 11th overall prospect, enjoyed a breakout 2021 in the minors with ridiculous strikeout rates and excellent control. He eventually made his eagerly anticipated debut, throwing 13.1 innings and having no problem translating his elite skills to the Majors. Don’t be sad to find his name second on this list, as his xK% was still an impressive 32.7%! That’s a massive S/Str, which more than offsets the low L/Str rate. The sample was tiny, but his three primary pitches, his four-seamer, slider, and curveball, all generated double digit SwStk% marks. His slider was even over 20%. It’s hard not to be excited about Baz, especially as his fastball averaged 97 MPH. It’s seemingly the stuff of a true future ace.

After a long minor league career and ups and downs from the Majors, Nestor Cortes is currently penciled into the Yankees rotation to open the 2022 season, whenever that may be. His 2021 results were impressive and likely make him a sleeper in many owners’ eyes. However, his xK% suggests he overperformed, and really, only looks a bit above average because of a high F/Str. His S/Str is below average and his minor league rates don’t give us much hope for better. Since he probably won’t cost too much, I won’t call him a potential bust, but I don’t expect him to make his new owners any profit. His extreme fly ball tendency is scary in Yankee Stadium.

Lance Lynn is an annual overperformer, as going back to 2015 (the earliest season on my file), he has outperformed every single year. However, this is the largest degree of outperformance in a season, as he has averaged 1.3% in outperformance previously. This was also his lowest xK% since 2018, and it has dropped each season since surging in 2019. I have never believed in Lynn, as he has always outperformed his underlying skills and is essentially all fastball, but has upped his cutter usage the last couple of years. He’s the type that will likely never end up on my team after the draft, as I prefer rostering pitchers who aren’t massively outperforming their SIERA every year. You just never know when that outperformance is going to end since it’s difficult to pinpoint what has led to the outperformance in the first place.

Finally, we got a full season from Michael Kopech! It wasn’t a full season in the rotation, but in the bullpen, and he didn’t disappoint. That said, his strikeout rate was a bit over his head, but still, an xK% over 30% is plenty good. I would have liked to see his highly graded slider generate a higher SwStk%, but his four-seamer was elite, which is a great sign after returning from TJ surgery. His value will highly depend on whether he opens the year in the rotation or bullpen.

Injuries have wrecked Stephen Strasburg the last two seasons and now it’s anyone’s guess how he’ll perform when he returns to the mound. Thoracic outlet surgery has not been kind to the small sample of pitchers who have returned from it, so I’m not particularly optimistic here.

It was one of the most surprising breakouts last year, but Carlos Rodon returned from injury after injury to post just his second sub-4.00 ERA and first sub-3.00 ERA, with a sub-3.00 SIERA as well. It was backed by a big jump in fastball velocity. Most of the spike was real, so the takeaway here is more about that 32.5% xK%, rather than the fact he slightly overperformed it. He dealt with more arm issues during the season though and is now a free agent, so he’s quite the risk/reward pick this season.

Like Lynn above, Gerrit Cole has also consistently overperformed his xK%. His 2021 season wasn’t even his largest overperformance, as he did so by 3% in 2019. Well, of course, as he posted an actual mark of 39.9%! It’s highly, highly unlikely that xK% is going to give you that credit over more than 200 innings. Since Cole’s big strikeout rate surge in 2018, this was his lowest xK%, but just barely, being only 0.1% lower than in 2020. Still, these last two seasons were meaningful xK% drops versus 2018 and 2019, which shouldn’t be surprising considering how hard it is to maintain a mid-30% xK%.

Continuing the theme, Max Scherzer has also regularly overperformed his xK%. The bottom line is that just like any equation, it’s going to struggle at the extremes. This was Scherzer’s largest overperformance going back to 2015 though, but that’s nothing to worry about as his xK% has remained remarkably consistent in the low-30% range the entire time. At age 37, he’s still going strong.





Mike Podhorzer is the 2015 Fantasy Sports Writers Association Baseball Writer of the Year. He produces player projections using his own forecasting system and is the author of the eBook Projecting X 2.0: How to Forecast Baseball Player Performance, which teaches you how to project players yourself. His projections helped him win the inaugural 2013 Tout Wars mixed draft league. Follow Mike on Twitter @MikePodhorzer and contact him via email.

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jbgocubsmember
4 months ago

not sure how to properly word this, but I feel like an expected statistic that has such a tiny separation from actual performance across the board (maximum overperformer of 4.5%, maximum underperformer of 3.4%) maybe is not as advanced a stat as some would like to think.

jbgocubsmember
4 months ago
Reply to  jbgocubs

half these blurbs are something along the lines of “yeah, this guy has showed up as an overperformer every year since i created this stat.” I don’t know man, I think we need to tinker with the formula a bit!

Iramember
3 months ago
Reply to  jbgocubs

Also, with the exception of Strasburg the difference is almost irrelevant because the expected K% is still huge. Does it really matter when the figure is still way above league average?

Joe Wilkeymember
3 months ago
Reply to  jbgocubs

I have always thought that about this formula, there’s too many variables to take any action off of it. If you look at the post for the most recent version of this (https://fantasy.fangraphs.com/introducing-my-hitter-xk-version-2-0/), there are some pretty big correlations between variables in a couple of cases.

If you’re looking for something a little simpler and with potentially more actionable information, may I suggest my article? (https://community.fangraphs.com/looking-at-xk-and-xbb-using-statcast-zones/)