AL Starting Pitcher Strikeout Rate Upside

Earlier this year, I updated my pitcher expected strikeout rate metric, or xK%, with new coefficients. The equation uses a pitcher’s overall percentage of strikes thrown, as well as the breakdown between the types of strikes he generates — swinging, looking, and foul. We could use xK% over a smaller sample given that its denominator is pitches, rather than batters faces, so it likely stabilizes much more quickly.

Let’s therefore take a look at the AL starting pitchers whose xK% marks are significantly higher than their actual marks, suggesting potential for a serious strikeout rate spike.

Since strikeout rate is a relatively stable stat, especially after nearly half a season’s worth of starts, we don’t see enormous differences between xK% and K%. But there are certainly some discrepancies which could provide some actionable information.

I literally just included Dylan Bundy on Tuesday’s list of pitchers whose SIERA marks are significantly higher than their ERAs, suggesting real ERA regression potential. But what SIERA and all the ERA estimators assume is that the underlying skills from which the equations are driven by are stable. Obviously, that’s rarely the case. Bundy may be fortunate to own a sub-4.00 ERA now given his weak strikeout rate, but xK% suggests that he actually deserves to have punched out far more batters. The 21.1% xK% is pretty close to last year’s mark, so it passes the sniff test. Given his extreme fly balls ways, he’ll need to up the strikeout rate to maintain an ERA below 4.00.

If you thought Jordan Montgomery has been good, well, perhaps he should be performing even better. Out of the 60 pitchers in my population, he ranks sixth in swinging strike rate. Unfortunately, both the looking and foul strike rates are below average, but he does throw lots of strikes. Amazingly, he has thrown three secondary pitches that have generated excellent double digit SwStk% marks. It reminds me of Jacob deGrom’s breakout with his assortment of strong offerings that seemingly provided a high floor. I’ve been skeptical here, but maybe he has some real staying power.

You know who is second, just behind Chris Sale in S/Str rate? Sean Manaea. While his fastball velocity is down from last year, the pitch’s SwStk% has spiked, while both his changeup and slider have been magnificent, generating SwStk% marks above 20%. Control has been his only issue, as his strike percentage is just barely above the league average.

Hmmm, so while Kevin Gausman has clearly been bad, very bad, maybe he deserves a slightly less bad fate. Earlier in the season, I wondered why his splitter usage was down since it’s clearly his best pitch and a really fantastic one at that. Finally, he recognized my bewilderment and over his last three starts, has been throwing the pitch around 20% of the time. Of course, in just one of those three games was his SwStk% anything to write home about. He’s a mystery, though it’s clear he needs to throw more strikes (Str% way below league average).

If Jason Vargas could bump his strikeout rate up closer to his xK% mark, it would certainly stave off some of the ERA regression he faces once his HR/FB rate and LOB% normalizes. He induces a shockingly low rate of foul strikes, but lots of called strikes, and an awesome changeup that makes up for a weak fastball.

Gee, I don’t think fantasy owners were thinking 20.5% was Danny Duffy’s strikeout rate upside. He’s still making batters swing and miss (though less often than last year), but bizarrely his looking strike rate has now dropped for a fourth straight season. I have no idea what that means. His strike percentage has also regressed back to his 2014/2015 levels after a surprising surge in control last year…which is proving to have been a fluke.

My historical xK% file goes back to 2011, and this is the second lowest S/Str mark for Justin Verlander since then. He has made up for it some with a surge in foul strike rate, but what happens when some of those former foul balls get put into play? He owns a far less stable xK% when it’s driven mostly by an inflated F/Str. Even with his velocity up, his strikeout rate is a far cry from last year. But then again, he’s 34, so it would have been silly to expect a repeat.