2018 Midseason MLB Analysis - Starting Pitcher Risers and Fallers in K%

As we reach the non-official midpoint of the MLB season, now seems like as good a time as any to take a look at some pitchers who've seen the most dramatic upticks or downslides in their strikeout percentages. We'll take a look at what could be causing the changes and whether their first half numbers could continue for the rest of the season.

Also be sure to check out our MVP, Cy Young and Biggest Surprise picks for the first half.

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Biggest Risers

Gerrit Cole - HOU
2018: 35.2 K%
2017: 23.1 K%
 Diff: 12.1%
The Astros made a splash in the offseason trading a haul of prospects (with Musgrove at the core) for Cole. The latter was coming off a solid season that saw him post just under a strikeout an inning with an xFIP right at 3.81. He was a solid major league starter with pedigree, but those numbers wouldn’t blow anyone away. This season? His numbers are blowing people away. He’s made such a tremendous leap with the strikeout stuff, completely outpacing his previous best K% season (24.3% back in 2015). It helps moving to one of (if not) the best analytics teams in all of baseball. The Astros and Cole have reconfigured his approach, essentially ditching the sinker and changeup (he threw them 18% and 11% of the time respectively last season) in favor of a slider and curveball combo to compliment the fastball. It’s not that he didn’t throw those pitches last season, it’s just that he’s throwing them more now. It’s working. He’s posting the second-highest K% in the majors behind Chris Sale (37.2%) and because of the new approach, I see the number sticking in this range. It’s always tough, from a projection standpoint to assume a single season’s gains are sustainable after a big sample size that would suggest otherwise, but I think it’s the case with Cole. According to FanGraphs linear pitch values, the slider and curveball have been devastating to opposing hitters and have only served to strengthen the fastball. It’s all working and it sure seems like Cole will stick with the elite tier of strikeout guys in the majors.

Patrick Corbin - ARI
2018: 30.4 K%
2017: 21.6 K%
Diff: 8.8%
Speaking of players using a new approach to outsized strikeout gains, Patrick Corbin is enjoying the best season of his major league career (and it really isn’t all that close). Maybe you want to credit the humidor installed in Chase Field that appears to be significantly reducing power in the once-hitter’s park. That has to certainly be helping Corbin in terms of attacking the zone and induce more ground balls. He’s close to elite in that arena with a 50% GB% over the last two seasons. But he’s also instituted major changes in his repertoire. Over the course of his career, Corbin threw the fastball about 30% of the time. This season he’s down to 20%, making up the difference with an uptick in sinker offerings as well as pulling a curveball completely out of nowhere. According to FanGraphs Pitch F/X data, Corbin had never thrown a curveball in the majors. Now he’s throwing one 10% of the time, in place of the changeup which he’s basically ditched. This kind of shift means teams likely haven’t had a chance to fully make the adjustment to the new offerings, considering he’s seen a rise in O-Swing% and a reduction in overall Contact %. Hence the spike in strikeouts. Looking at the recent game log, and the idea that teams will continue to adjust to the new approach, I don’t think we see Corbin stick in the 11 K/9 range for the second half, but finishing at something like 25-26% K% by the end of the season is definitely in play.

Mike Foltynewicz - ATL
2018: 28.9 K%
2017: 20.7 K%
Diff: 8.2%
Folty is the other big gainer in strikeout stuff this season, with his best season of swing and miss offerings we’ve seen from the young righty. We’ve got some approach changes like the two guys above (will get into those in a second) but unlike Cole and Corbin, Folty has something else going for him: he’s throwing harder. Last year, the fastball hovered around 96 mph, but this year it’s up to a steady 97 mph. And it’s not just the fastball. He threw the sinker at 95.7 mph last season, but that’s up 1.8 mph to 97.5. It’s helped make, according to Pitch Value info, all of his pitches better and really sets up the slider which is now, by far, his best offering. That pitch hasn’t added any velocity, but he’s throwing it more than ever. According to that data, Folty has the fourth-best slider in the league. With the increase in velocity, it's also meant him throwing the fastball more along with a 5% increase in the aforementioned slider.

Honorable mentions
Derek Holland 2018 - 24.1 K% , 2017 - 16.6 K%, Diff - 7.5%
Justin Verlander 2018 - 32.5 K% , 2017 - 25.7 K%, Diff - 6.7%

Biggest Fallers

Corey Kluber - CLE
2018: 25.8 K%
2017: 34.1 K%
Diff: -8.3%
The good news is, he’s still been a very good pitcher thanks to a fine enough strikeout rate and a dip in walks (3.3% which is absurdly low). He’s also got an ERA under three and a 3.05 xFIP. So Kluber, by and large, has been fine. But he has seen the single biggest season-over-season dip in strikeout rate. My takeaway here is that, rather than it being an “issue”, it more looks like the 2017 K stuff was an outlier. The 26% K% is right in line with what he’d put up in 2016 and 2015 (27.7% and 26.4%). Last season he threw the slider more than any other time in his career while throwing the sinker less. He’s reversed course in 2018, essentially going back to his 2016 repertoire with the one exception being the lack of four-seam fastballs in favor of the cutter. I think we see Kluber sticking right around this K per inning stuff simply because that’s what he’s been for most of his career (with a moderate decline every year until last year). In his age 32 season, I don’t think we see a late-season spike in the K’s unless there’s a possible concentrated effort to return to last year’s pitcher profile. I don’t think that will be the case.

Jake Arrieta - PHI
2018: 17 K%
2017: 23.1 K%
Diff: -6.1%
As he’s lost velocity over the last few seasons, we’ve seen Arrieta get away from trying to dominate hitters with an upper 90’s fastball. Instead, he’s become, almost exclusively, a sinker/ slider guy. He used to throw the fastball 20% or more, but he’s used it less and less over the last four seasons and in 2018 has only thrown the *heater* 3% of the time. He’s strictly a groundball pitcher now, with a fantastic 55.3% GB% helping to mitigate the loss of the strikeout stuff. It’s tough to tell if it’s chicken or the egg here, but as he’s lost velocity on the fastball he’s made adjustments to become a different type of pitcher. Namely, one who can hang around in the majors without overpowering stuff. It *works* in that he’s an above-average arm, but 2014-2015 Jake Arrieta ain’t walking back through that door.

Dishonorable mention
Jose Quintana 2018: 20.8 K% , 2017: 26.2 K%, Diff: - 5.4%

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