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Changes in the slider helped Kelvin Herrera return to a serviceable level

Herrera made changes to his slider in 2018 that attributed to his improvement, but all the hope lies in the fastball.

Baltimore Orioles v Washington Nationals Photo by Rob Carr/Getty Images

Yesterday afternoon, the White Sox made a move to bolster their bullpen (or possibly farm system, depending how you look at it), signing veteran reliever Kelvin Herrera to a two-year, $18 million deal. Years ago, prime Kelvin Herrera looked like one of the more talented relievers in all of baseball, giving him hope of a lucrative long-term deal in the future if his heath would persist.

But entering free agency for the first time in his career was rather untimely, as he was coming off year-to-year wobbly performances and a season ending Achilles injury he sustained in September. Now viewed as an upside tier-two reliever, his market looked sort of perplexing with the injury factored in. The White Sox saw an opportunity to buy-low and pounced.

Looking at the underlying numbers and peripherals, you would come to the conclusion that Herrera is a reliever hitting the falling end of the aging curve, a lot of which is true. His strikeout-rate has fallen the past two seasons. His walk-rate has fluctuated the past few seasons. His ground-ball rate took a major tumble to a career-low mark in 2018. The ERA and FIP took a severe hit as the season went on.

Dive deeper though and you’ll find that his xwOBA in 2018 was the second-lowest mark of his career in the Statcast-era (top third in baseball). His exit velocity allowed sat at a very impressive 86.7 mph, and despite the below-average strikeout-rate, a feat rather unusual for a pitcher of his profile, he was still inducing whiffs at an encouraging rate. The fastball velocity, though declined, was still comfortably above 95 mph, and his secondaries remained effective.

You can spin it multiple ways, but it would be totally fair to assume that Herrera is a declining, yet still very effective reliever with some degree of upside.

Notably, Herrera made a significant change to his pitching style in 2018. He took 6.8 mph off of his slider from 2017, more than any other qualified reliever in baseball.

Factoring the slight decline in four-seam velocity, Herrera saw a large increase in velocity separation between his four-seamer and slider. Sitting at a 9.3 mph differential in 2017, he ranked 45th out of 80 relievers with at least 40 innings in both 2017 and 2018. Among that same group in 2018, he ranked first in differential, sitting a full mph ahead of Kyle Barraclough.

Biggest Changes in Four-Seamer/Slider Velocity Separation

Name 2017 Differential 2018 Differential Diffferential Change
Name 2017 Differential 2018 Differential Diffferential Change
Kelvin Herrera 9.7 15.6 5.9
Jacob Barnes 6.5 10.6 4.1
Ryan Tepera 10.3 13.8 3.5
Seung Hwan Oh 7.4 10.2 2.8
Shane Greene 11 13.7 2.7
Archie Bradley 5.3 8 2.7
Adam Morgan 11.2 13.1 1.9
Joe Kelly 8.6 10.3 1.7
Chris Rusin 7 8.6 1.6
Jose Leclerc 12.4 13.9 1.5
Between 2017 and 2018 FanGraphs

Much of Herrera’s struggles in 2017 went back to his slider. Hitters went from a 53.3 percent K-BB-rate with a .133/.133/.200 slashline in 2016 to a 26.1 percent K-BB-rate, .286/.304/.619 slashline in 2016. It was straight and hitters weren’t missing it.

By taking off velocity in 2018, Herrera saw a dramatic increase in spin rate (2490 vs 2692).

Baseball Savant
Baseball Savant

If you’re wondering where the decline in strikeouts came from, considering the slider was improved, it lies in the fastball. Herrera has consistently hovered around a 20 percent strikeout-rate with the four-seamer for his career. In 2018, it plummeted down to 12.6 percent. At this point, it looks like he’s traded whiffs for control and contact-management, which can be very concerning for an aging reliever.

Herrera seems to be one fastball adjustment away from returning to his dominant self. The velocity is still there, albeit declining. He still has the complimentary secondaries. It wouldn’t be something to bet on though, which is where the questioning can come in this free agent deal.

All in all, though, this signing looks fine for the White Sox. They should have multiple opportunities to trade him if he continues to perform at at least a serviceable level, and as mentioned above, there is still upside for Herrera to return to an impact arm, possibly extending any possible return.

Patrick Brennan loves to research pitchers and minor leaguers with data. You can find additional work of his at Royals Review and Royals Farm Report. You can also find him on Twitter @paintingcorner.