Despite A Rocky Season, Zack Kelly Could Be A Weapon Out of the Red Sox Bullpen in October

Zack Kelly is not a name that Red Sox fans are terribly fond of. A 30-year-old reliever, Kelly has pitched in parts of four seasons with the Red Sox, dealing with injuries while pitching in both Triple-A and the major elagues. His most prominent season with Boston was in 2024, when he appeared in 49 games in what was a season of two halves. In 25 first half appearances, Kelly went 3-1 with a 1.78 ERA in 35 1/3 innings where he struck out 39 batters. In the second half, though, it all seemed to fall apart for the right-hander, as he appeared in 24 games and went 3-2 with a 7.59 ERA in 21 1/3 innings while striking out just 22 batters.

The 2025 season has been more of the same for Kelly, as he’s appeared in 17 games for the Red Sox and has gone 1-3 with a 5.56 ERA in 22 2/3 innings. In that span, he’s allowed 14 earned runs on 24 hits, two home runs, and walked nine batters while striking out 25. Kelly broke camp with the Red Sox to begin the campaign, but its been a constant shuffle between Worcester and Boston as he’s been optioned to Triple-A three times on the season, along with missing significant time due to an oblique injury. 

Despite all of that, he might be better than you think.

Kelly’s FIP is actually a much more respectable 3.64, and in conjunction with his expected ERA of 3.65, there's reason enough to believe that he’s getting extremely unlucky at times on the mound. Case in point: opposing batters have a batting average on balls in play of .344 this season against Kelly. As such, they are hitting .264 against him, despite an expected batting average of just .236.

But what makes Kelly most intriguing for a playoff run is his ability to limit hard contact to go along with high strikeout numbers. While he doesn’t have enough innings to qualify for it, Kelly would be near the top-third of the league right now for average exit velocity, barrel percentage and hard-hit rate, sitting at 87.8 mph, 6.1% and 36.4%, respectively. Add to it a strikeout rate of 24.3% while striking out nearly 10 batters per nine innings (9.93), and Kelly clearly has the stuff to dominate out of the bullpen. The cherry on top is his continued success in limiting home runs; the right-hander has allowed 0.79 home runs per nine innings this season, which comes out to just two allowed in his 22 2/3 frames.

Of course, his ERA of 5.56 and walk rate of 8.7% can't be ignored, nor can his penchant for allowing nearly half the batters who reach base against him to score (42.8% of them). However, if there’s anything that plays well in October, it’s velocity, and Kelly’s fastball averages 96.1 mph. The Red Sox are no stranger to this either, having dealt with similar circumstances in the lead up to their 2018 World Series championship.

Joe Kelly was a 30-year-old reliever who had his ups and downs in 2018 that some believed should have been left off the postseason roster entirely. In 73 games that season, he only pitched 65 2/3 innings and had a 4.39 ERA while allowing 32 walks and striking out 68 batters. Kelly had a walk rate of 11.2% but thanks in part to his velocity (98.1 mph fastball and a 97.6 mph sinker), he made the postseason roster. That proved to be a smart call, as he held batters to a .186 batting average while he appeared in nine games and tossed 11 1/3 innings while allowing just one run to score.

Heath Hembree is another example of velocity playing in the postseason as he struggled at times in 2018. Appearing in 67 games that season, Hembree tossed 60 innings and had a 4.20 ERA. Despite that, he still pitched in four games during the postseason where he didn’t allow a single hit or run. While he wasn’t as big of a part of the championship effort as Joe Kelly, Hembree provided 4 2/3 quality innings for the bullpen when called upon.

Zack Kelly is much better than his stats show, and if he can be even half the pitcher he was in the first half of 2024, then the Red Sox could have another dominant piece for their bullpen in October. It would be a risky play, but Kelly could be the final piece they need to lock down games once the postseason rolls around.