xK% and Potential Pitcher Strikeout Rate Surgers

Two years ago, I modified the first equation I developed to yield an improved expected strikeout rate formula. The formula uses a trio of strike type rates found at Baseball-Reference.com, including a pitcher’s looking, swinging, and foul strike percentages, as well as his overall rate of strikes thrown. The beauty of the equation is that it uses components that stabilize quickly, as the rates as per pitch, rather than per inning or per batter. I calculated the xK% marks for all qualified starting pitchers, compared it to their xK% marks, and sorted. Let’s discuss those with the most significant potential upside, as suggested by xK%.

Potential Strikeout Rate Surgers

Name	SIERA	K%	xK%	Diff
Martin Perez	5.41	11.4%	16.7%	-5.3%
Cole Hamels	3.99	22.3%	26.6%	-4.3%
Steven Wright	4.06	22.5%	26.5%	-4.0%
Yordano Ventura	5.63	18.3%	22.3%	-4.0%
Clay Buchholz	5.08	15.3%	18.5%	-3.2%
Derek Holland	4.75	15.5%	18.5%	-3.0%
Jordan Zimmermann	4.06	17.4%	20.3%	-2.9%
Alex Wood	4.21	16.7%	19.5%	-2.8%
Chris Young	4.04	23.2%	25.9%	-2.7%
Scott Kazmir	4.01	19.1%	21.5%	-2.4%
Matt Harvey	4.32	16.9%	19.2%	-2.3%

Martin Perez returned from Tommy John surgery last season and his fastball velocity currently sits at its highest marks since 2013, which is a great sign. Furthermore, his SwStk% is also at its highest perch, but you wouldn’t know that by just looking at his strikeout rate. He has walked more batters than he has struck out, so his control has regressed. But that should get better, and with his high ground ball rate, the xK% and better velocity means he could squeeze out some AL-Only league value.

Cole Hamels has a shiny 2.52 ERA, but his underlying skills have deteriorated and his SIERA sits at a career worst mark. xK% suggests that the strikeout rate decline shouldn’t be a concern as he should enjoy a bump, but what is troublesome is a complete loss of control. His strike percentage has tumbled below the league average to a career low, and it has been a precipitous falloff. There have been no whispers about injury, but after 411 pitches, it’s not something to just ignore. If I were a Hamels owner, I’d be a bit nervous that something isn’t totally right.

Knuckleballing Steven Wright has made a strong case to remain in the rotation even after the Red Sox get their injured starters back. And while there is a substantial gap between his 1.37 ERA and 4.06 SIERA, remember that knuckleballers don’t follow the same rules and xK% suggest an even higher strikeout rate, which would push his SIERA below 4.00.

Forget about Yordano Ventura’s apparent strikeout rate upside. Although it’s a good sign not to be too concerned by the sub-20% strikeout rate, that inflated walk rate is the major red flag. His strike percentage is below 60% (league average around 64%), down about 2.5% from last year and 3% below his career average. He missed time in 2014 with an elbow issue and last year with inflammation in his forearm. Uh oh. Where there’s smoke, there’s fire. Since his ERA sits at a respectable 3.67, I would be racing to sell if I were an owner. Hopefully your potential trade partner doesn’t notice the 20 walks and just sees the ERA and the name with the big upside.

I discussed Clay Buchholz here and he proceeded to pitch poorly again in his next start, though his underlying skills were bad. No poor luck this time! As an AL Tout Wars owner, the only thing I can do is cross my fingers and hope things get better. I wouldn’t buy low though.

Derek Holland once was an intriguing young pitcher, but injuries have taken their toll, as they often do. Now he has become a middling southpaw, with a slight fly ball tilt, weak strikeout rates, and only good control his one saving grace. xK% hints at better things ahead, but that would mean that his rest of season ERA is in the low 4.00 range, rather than mid-4.00 range. With an ERA below 3.00, sell him for anything you can get.

Who said that moving to the American League would destroy Jordan Zimmermann’s ratios?! He’s back to his magical act, as he is once again outperforming his SIERA by a hefty margin. Obviously, it won’t continue at this rate, and not surprisingly, but it’s a good sign that xK% doesn’t think his strikeout rate should have lost anything just yet, despite the move to the AL. Still, these are meh skills and it makes him a sell candidate.

Increased fastball velocity hasn’t done anything for Alex Wood’s strikeout rate, but his strike percentage does suggest better control is imminent. He has posted a big ground ball rate, which is encouraging. I have no doubts that he’ll earn positive value in NL-Only leagues and there’s a glimmer of hope that he may even become relevant in 12-team mixed leagues.

So Chris Young enjoys a strikeout rate surge, which xK% suggests should be even surgier, his SIERA matches his career best mark, and yet his ERA sits above 6.00. This is what happens when his BABIP and HR/FB rate regress. The strikeout rate spike is the result of an increase in his slider usage, as he has thrown the pitch more than half the time! It has led to more strikeouts, but taken a bite out of his fly ball and pop-up rates, which have helped to push his BABIP into more human territory. Since he has proven to be extremely unique and essentially impossible to project, I’m not sure which version of Young is best. However, it’s clear that his results should improve dramatically.

The spring training velocity scare was an overreaction, as Scott Kazmir’s velocity is fine. His injury history and perennially second half slide has scared everyone away, and although his 5.00+ ERA is going to come down, I don’t find him a particularly strong acquisition target.

WHAT IS GOING ON WITH MATT HARVEY?!?! His fastball velocity is down a mile and a half and all of his pitches are at SwStk% marks that represent career lows. That’s troubling. Every pitch has generated a lower rate of swings and misses! Nobody cares that xK% says it should be a bit better, because even a 19.2% strikeout rate would be a major disappointment. Maybe the velocity suddenly returns and he’s back to vintage Harvey, but I would be too nervous to try buying low here.