xK% and Potential Pitcher Strikeout Rate Decliners

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.

Yesterday, I discussed the starting pitchers whose xK% most exceed their actual strikeout rates. Today, I’ll look at the other side of the list — those starting pitchers whose actual strikeout rates most exceed their xK% marks. These pitchers are at significant risk for regression.

Potential Strikeout Rate Decliners

Name	SIERA	K%	xK%	Diff
Jerad Eickhoff	3.11	25.2%	19.7%	5.5%
Rick Porcello	2.80	28.1%	23.8%	4.3%
Chris Archer	3.36	29.5%	25.3%	4.2%
Carlos Rodon	3.65	22.9%	18.9%	4.0%
Raisel Iglesias	3.48	23.6%	20.0%	3.6%
Gio Gonzalez	3.48	24.3%	20.7%	3.6%
Patrick Corbin	4.55	14.3%	10.7%	3.6%
Adam Conley	3.90	25.5%	22.0%	3.5%
Vincent Velasquez	2.89	32.0%	28.6%	3.4%
Jose Quintana	3.26	26.5%	23.1%	3.4%
James Shields	4.91	15.7%	12.4%	3.3%

Jerad Eickhoff was a popular sleeper heading into the season and for good reason — last season his slider was outrageous and complemented it with a strong curve as well. Unfortunately, both pitches have lost some swing and miss. And yet, his strikeout rate is up so far! Furthermore (and this will be covered more in a future post), his strike percentage is below average, and yet he has walked just 3.9% of the batters he has faced. What’s interesting is that his SIERA sits a full run below his actual ERA, making him look like a prime acquisition target. However, I’d argue his peripherals are due for a decline, perhaps a precipitous one, so his SIERA is going to jump and narrow the gap between it and his ERA, rather than the ERA plummeting toward his SIERA. All that being said, I still think he’s someone to keep around in 12-team mixed leagues.

What has gotten into Rick Porcello?! After last year’s strikeout rate surge, he has enjoyed another spike so far. But what’s driving it is not an increase in whiffs induced. Instead, it’s a career high looking strike rate. It makes me much more skeptical that he could maintain that xK%, which already suggests his actual strikeout rate is way above his head. However, even with regression, he should remain an asset in mixed leagues, assuming he doesn’t receive the terrible defensive support he suffered from last year.

Chris Archer’s inability to throw strikes like he used to is hurting his xK% and making his actual strikeout rate difficult to sustain. With his control problems, a drop in strikeout rate is going to be problematic, so he needs to right the ship soon.

Sharpening his control seemed to be the only thing standing between Carlos Rodon and a major breakout. And while his walk rate suggests better control, his strike percentage has barely inched up. The problem here is that his swinging strike rate has tumbled, thanks to a less whifftastic slider. Also worth noting is that his fastball has lost a couple of ticks, though that hasn’t affected his fastball’s SwStk%. Perhaps the effect is coming out in his slider’s effectiveness. The ground ball rate increase is also encouraging, so there is at least one positive sign. He’s a tough one to get a read on now.

Raisel Iglesias is now out with a shoulder injury, which probably explains the loss in fastball velocity and strikeout rate, though xK% thinks that strikeout rate should be even lower. So far, his overall luck had turned after he posted an ERA nearly a run higher than his SIERA last year. It’s yet another reminder that we should start at SIERA when forecasting future ERA, not ERA itself.

Gio Gonzalez’s strike percentage is at its lowest mark since 2011 and his velocity is down more than two miles per hour. Sounds like a disaster so far, huh? Nope! He has managed to post a microscopic 1.42 ERA, thanks to great fortune all the way around. With a walk rate not supported by his strike percentage, and strikeout rate ripe for regression, along with the velocity loss, he looks like a prime sell candidate.

Patrick Corbin made a triumphant return from TJ surgery last year, but everything has turned South this season. His strike percentage has dropped, and his swinging strikes have disappeared. His velocity, while down a bit from last year, is in normal territory and certainly not at a level to raise a red flag. But the four-seamer is just not getting whiffs, and his typically elite slider has been less elite this year. I don’t know what’s going on, but when a pitcher struggles after returning from major elbow surgery, you have to wonder. I would be nervous as an owner and would shy away from trying to acquire him.

I’m not sure how this happens so frequently, but reports of Adam Conley’s velocity bump during the spring appeared to be exaggerated. Sure, his velocity is up a bit, but nowhere near where we thought it might be. However, both his changeup and slider have induced more swinging strikes this year, but he has simply stopped throwing strikes, which would explain his inflated walk rate. If he improves his control, the xK% would rise and come close to his actual strikeout rate. I’d hold him in all leagues.

Vincent Velasquez’s appearance here is just a reflection of the limitations of a formula. By nature, a regression equation isn’t going to handle the extreme cases well. His xK% is actually 10th highest in baseball and his swinging strike rate is highest among pitchers on the above list. He has legitimately been awesome. Oddly, neither of his secondary pitches in the changeup and curve have been all that great at inducing swings and misses. It has been his four-seamer that has been elite. It doesn’t seem as sustainable as if he has had such success with his off-speed stuff. Given the guarantee of an innings limit, I would explore selling.

Jose Quintana has seemingly gone from boring to eye opening. His pitch mix is essentially the same, as is his fastball velocity, but he’s getting more swings and misses from the fastball and changeup. I don’t know why. My knee jerk reaction is to figure it’s a fluke and expect his strikeout rate to return to his previous level. Even with the spike in SwStk%, none of his pitches are generating marks above 11%. Basically, he has no plus pitches in his arsenal, it’s just a whole lot of mediocre stuff. It’s why the “boring” label has been slapped on him in the first place, and he’s not doing enough for me to remove it.

What has happened to James Shields?! His strike percentage not only hasn’t rebounded off last season’s decline, but it has sunk even further to a career low. Combine that with a career low swinging strike rate and disaster ensues. The crazy thing is that xK% thinks the strikeout rate, which is already shockingly low, should be even lower! But Shields has magically posted a 3.55 ERA, thanks to a .232 BABIP, to mask such a precipitous decline in skills. Given the still respectable ERA, it might make sense to try swapping him for another starter that seemingly has much better prospects the rest of the way.