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Meet Junior Guerra, reliever

Another flexible reliever for the Brewers!

Japan v MLB All Stars - Game 5 Photo by Kiyoshi Ota/Getty Images

The bullpen has saved many major league careers. Wade Davis, Andrew Miller, Dellin Betances, Jeurys Familia, Mariano Rivera (!) etc, it’s been iterated time and time again.

Higher velocity, more flexibility with a repertoire, saved energy can completely shift a pitcher’s performance in a certain role. With all these possible changes, most pitcher that makes the change from starter to reliever can become an intriguing case.

And I think that I may have found that next intriguing case! Sitting in the Brewers bullpen set to fulfill the swingman role to start the 2019 season is the 34-year-old former journeyman starter Junior Guerra. A nice story for his career, Guerra bounced around a few organizations to start off, had a seven-year absence from affiliated ball, made a couple more organizational stints before winding up in the 2016 Brewers rotation. The wait was well worth it, as Guerra became one of the better storylines of the season, posting a 2.81 ERA and 3.70 FIP across 20 starts. Missed time and inconsistencies in command made the follow-up season less exciting though (5.12 ERA in 70 1⁄3 innings).

Despite the struggles, Guerra ended up with a spot in a depleted Brewers rotation for the 2018 season, actually pitching well throughout most of the season (96 IP, 3.23 ERA, 96 SO, 39 BB in the first half), before running out of gas post-break (43 13 IP, 6.02 ERA, 40 SO, 16 BB). September came, followed by an extended Brewers postseason run, and Guerra was relegated to a bullpen role.

Performing further examination into Guerra’s bullpen splits admittedly comes with a sketchy sample size. He’s only pitched 16 13 innings of relief for his major league career (3.31 ERA, 23 SO, 3 BB). Six of those innings came last September.

The good news is, nothing in this small case says he can’t become a good reliever. If anything, there’s intrigue. We can also look further into Guerra’s relief experience by adding in last October, another small burst where he excelled in.

Guerra’s main secondary offering throughout his career has been his slider. It’s never been a particurlay amazing his pitch, but it supplements is repertoire, consistently sitting in the 81-83 MPH range as a starter. But when Guerra moved to the bullpen, something interesting happened to that pitch. As a reliever, he dipped the velocity on his slider, perhaps giving it more separation to his four-seamer, which was predictably seeing an increase in velocity.

Mix in some small release point tweak (his arm angle saw a slight shift upwards), his slider started to look more like a slurvy breaking ball (Brooks Baseball actually started picking it up as a curveball!). This gave the pitch a significant boost in vertical movement.

Junior Guerra slider movement as a starter
Baseball Savant
Junior Guerra slider movement as a reliever
Baseball Savant

And the results went as followed... more strikes, more whiffs, more out-of-zone success, more overall production.

Junior Guerra slider production by role

Player Season Role wOBA xwOBA SwStr% O-Swing% Called Strike%
Player Season Role wOBA xwOBA SwStr% O-Swing% Called Strike%
Junior Guerra 2018 SP 0.307 0.295 14.6% 22.9% 16.8%
Junior Guerra 2018 RP 0.052 0.102 17.2% 32.3% 27.6%
Baseball Savant

And perhaps as an encouraging sign for future success, these stellar results came at a point of where he was throwing his slider at a higher rate than he has at any point in his career. He threw the pitch at a 42.3 percent rate in September. The next highest rate for a month in his career was September of 2017 when he threw it 22.2 percent of the time.

Baseball Savant

The slider clearly seems like the key to success for Junior Guerra’s production in a relief role, whether he ends sticking in his projected swingman role, moves into more situational outings, or as a middle reliever. Knowing how the Brewers operate with their bullpen, it seems Guerra would fit the bill well as a flexible arm. If he takes his small sample size of success and moves forward with it, that’s the cherry on top. And it would be another relief weapon for a team that already has a tremendous bullpen.


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.