Deep Dive: Aaron Bummer’s past, present and future with the White Sox

Top lefty: Aaron Bummer was one of the best southpaws relievers in the majors in 2019. (Clinton Cole/South Side Hit Pen)

“Deep Dive” focuses on the depth of each position in the Chicago White Sox organization. Each position is broken into a five-part series:

  1. Depth in the rookie levels (Dominican through Great Falls)
  2. Depth in A-ball (Kannapolis and Winston-Salem)
  3. Depth in the higher levels (Birmingham and Charlotte)
  4. Under the Radar-type detail on one of the White Sox players at that position
  5. Free agent options at that position

This article delves into the career of Aaron Bummer through 2018, his 2019 season with the White Sox, and what his future looks like in the White Sox organization.

How did he get here?

Bummer did well in his three years with the University of Nebraska, despite the fact that many of his peripherals weren’t as strong as he would’ve liked. He was used exclusively as a reliever during his freshman, split bullpen and starting duties his sophomore season, and was a starter during his junior season. (Another starter on the Cornhuskers squad was future White Sox farmhand Kyle Kubat.)

In his junior season, Bummer enjoyed a 7-5 record in 15 starts spanning 89 innings. During those games, he allowed 92 hits (.275 OBA) and 35 walks while striking out 56. The White Sox saw enough potential in Bummer to select him in the 19th round of the 2014 draft. Bummer compiled a solid 2.45 ERA and 1.09 WHIP for Great Falls that year, as he allowed just 18 hits (.222 OBA) and six walks (6.7%) while fanning 28 (31.5%).

Bummer went in for minor elbow surgery after the season to clear out “loose bodies,” causing him to miss the beginning of 2015. During rehab it was discovered he had a torn UCL, so he went in for a subsequent Tommy John surgery which caused him to miss the rest of 2015 and much of 2016.

Bummer returned to the mound and threw just 16.2 innings in 2016 across multiple affiliates, while displaying the stuff and high strikeout totals that he showed in rookie ball. The 2017 season saw Bummer spend time with the three top affiliates, combining to post a 3.31 ERA and 1.31 WHIP in 28 appearances. In 49 innings, he allowed just 44 hits (.243 OBA) and 20 walks (9.8%) while striking out 54 (26.5%). His season was far from over. Bummer also spent significant time with the White Sox, but his numbers suffered due to control issues. In 30 appearances for the Sox totaling 22 innings, he compiled a 4.50 ERA and 1.27 WHIP by surrendering just 13 hits (.178 OBA) but 15 walks (16.5%) while fanning 17 (18.7%).

Bummer started the 2018 season with the White Sox but struggled enough to be sent down to Charlotte for two months. With Charlotte, he seemed to right the ship as he posted a solid 2.64 ERA and 1.24 WHIP in 30 2/3 innings, along with 27 hits (.235 OBA) and 11 walks (8.6%) while striking out 30 (23.4%). He did earn a call-up to Chicago in September and did relatively well, but his overall numbers were disappointing thanks to a lack of command. In 37 outings for the White Sox totaling 31 1/2 innings, Bummer relinquished 40 hits (.301 OBA) and 10 walks (6.9%) while punching out 35 hitters (24.3%).

With the White Sox in 2019

Bummer began the season on the Charlotte roster, as the White Sox opted to go with Jace Fry and Caleb Frare as their primary lefties. After five solid relief outings for the Knights, Bummer was promoted to the White Sox on April 28 and never looked back. It’s nearly impossible to believe that, in 58 appearances for the White Sox in many crucial situations, he wasn’t credited for either a win or loss and received just one save for his efforts. However, despite his minimal impacts on the traditional reliever leaderboards, Bummer was easily the most effective reliever on the White Sox staff. Bummer posted a 2.13 ERA and 0.99 WHIP in 67 2/3 innings by allowing just 43 hits (.184 OBA) and 24 walks (9.2%) while striking out 60 (22.9%).

Bummer has quite an impressive five-pitch arsenal for a reliever. With that said, he used his sinking fastball (95.6 mph average per Baseball Savant) a whopping 67.7% of the time, an increase of 12% from the year before). That’s actually a good thing, as hitters slashed just .195/.246/.268 against it. This pitch, more than any other, contributed to his amazing 72.1% ground ball rate — topped only by Yankees reliever Zack Britton. The use of Bummer’s 87.9 mph cutter nearly doubled from 10.8% to 20% this year, again with terrific results as opponents slashed just .087/.145/.109 against it. Bummer lowered the usage of his four-seamer (average 95.1 mph) from 10.4% to 8.6%, also for good reason, as hitters had their best success against him at .353/.422/.529. Bummer’s fourth-most frequent offering, an 84.9 mph slider, saw its usage drop from 19.2% to 3.2%, though hitters hit just .167 against it. Finally, Bummer reduced the use of his changeup (87.3 mph) from 3.8% to 0.6% — while the results were good with it, it’s too small a sample size to judge its effectiveness.

Despite missing a month of the season while in Charlotte, Bummer hd an outstanding season. His stellar work out of the pen gave him an incredible bWAR of 2.8. Considering each bWAR point is worth approximately $7.7 million on the free agent market per FanGraphs, along with his 2019 salary of $555,000, Bummer produced a net value of $21 million. Bummer won’t be arbitration-eligible until the conclusion of the 2020 season, and will be under team control through the end of 2024.

What does the future have in store?

Since Bummer appears to be locked in with the White Sox for quite some time, the only question for now would be his future role with the White Sox. While he doesn’t possess the ultimate strikeout rate which would suggest he could handle the closing role any time soon, it’s likely he could handle that situation in case an injury to present closer Alex Colomé should arise. Relievers are often volatile, in part due to the tremendous workloads they are burdened with. Bummer entered 58 games this year, many in high-stress situations.

It would help if additional lefties could be relied upon for 2020 and beyond. Fry and Frare both struggled with their control this year, and Josh Osich was inconsistent when needed and may not return next year. Jacob Lindgren had missed multiple years due to injury, and only pitched as high as Winston-Salem this year, while Bennett Sousa and Andrew Perez have both shown promise but have pitched a combined two games beyond Winston-Salem. If a reliable arm that throws strikes could be added to the 2020 bullpen, the White Sox would better avoid risking Bummer to a long-term injury.

Both lefties and righties hit worse than .200 against Bummer, so with his effectiveness this year in inducing grounders this year and keeping earned runs to a minimum, he likely was in significant demand during this year’s trade deadline. Fortunately, the White Sox resisted the temptation to trade Bummer, as his combination of low salary, years of team control and performance should make him a key contributor to future competitive squads on the South Side.

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Despite my entire family being Cub fans, I grew up listening to Harry Caray and Jimmy Piersall on the radio as I listened to the entire season of the South Side Hit Men of '77. While it's true I have a myriad of other interests (American history, classic literature, classic rock, classic country, blues, jazz, gardening, family,etc.--not all in that order), few things aside from family surpass my interest in all things baseball--especially the White Sox.

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Assuming health and consistency (yeah, I know), it seems like Bummer ought to be a lock and a lynch pin in the pen for the next few years. It’ll be nice to have somebody like that around.

Felix Perez
Felix Perez

The Andrew Perez you got when you click his name is the wrong one. WS Andrew Perez went To USF.


Good article and I think Fry can be a great lefty out of the pen for us. Just needs to work on his control a lot this offseason, he has the peripherals.