The Red Sox rotation gets all the attention, and rightfully so. It has drastically improved without any additions, though we’ve seen some regression in the last few weeks. But Boston’s bullpen is doing well, too, despite injuries and some new faces, including a couple of rookies. More than a few Red Sox relievers are making significant contributions — I’ve already written about Brennan Bernardino — including rookies Justin Slaten and Cam Booser, among the best rookie relievers in all of MLB.
Justin Slaten Is the Better of the Two Red Sox Rookie Relievers
Embed from Getty ImagesJustin Slaten, 26, acquired by Craig Breslow in the Rule 5 Draft, is the best of the two.
Slaten owns a 2.84 ERA/1.88 FIP/148 ERA+/2.92 xERA with four walks, 34 strikeouts, and one home run allowed over 31.2 innings.
Opposing hitters have a .229/.256/.297/.553 slash line against him. Right-handed hitters have a .603 OPS against, and left-handed batters own a .491 OPS against.
He’s done better at Fenway (1.17 ERA) than on the road (4.41 ERA).
How’s Slaten having so much success?
The Texas native induces tons of groundballs (49 percent), throws strikes (66.5 percent), and gets lots of whiffs (30 percent) and strikeouts (27 percent).
He also limits walks (3 percent) and hard contact.
The Red Sox rookie reliever has induced a 2 percent barrel rate, 87 mph exit velocity, 31 percent sweet spot rate, and 36 percent hard-hit rate.
His fastball is his best pitch. Opposing hitters have just a .172 average, a .172 slug, and a .189 wOBA against it, with a 24 percent whiff rate, a 32 percent strikeout rate, and a 26 percent hard-hit rate.
Among rookie relievers, Slaten ranks 3rd in innings pitched, 4th in FIP, 6th in strikeout to walk rate, and 12th in ERA and strike rate.
Among all MLB relievers, he’s 8th in FIP, 22nd in strikeout-to-walk rate, 24th in innings, 54th in strike rate, 61st in ERA+, and 69th in ERA.
He’s also in the 97th percentile in walk rate, 96th percentile in barrel rate, 85th percentile in xSLG (.330), 84th percentile in xERA, and 82nd percentile in whiffs. And he’s in the 78th percentile in strikeouts, 76th percentile in exit velocity, 67th percentile in hard-hit rate, and 53rd percentile in xBA (.241).
Opposing hitters can’t square him up.
Cam Booser Is Good, Too, in a Different Way
Embed from Getty ImagesCam Booser has been good, too, just in his way.
Over 24.2 innings, Booser owns a 3.65 ERA/2.89 FIP/115 ERA+/2.94 xERA with five walks, 30 strikeouts, and three home runs allowed.
He’s holding opponents to a .242/.277/.389/.667 slash line. Righties have a .666 OPS against, while lefties own a .668 OPS against.
He’s been better at home (2.53 ERA) than away (4.50 ERA).
How’s Booser getting it done?
The rookie lefty reliever induces a ton of swings and misses (30 percent), chases (39 percent), and strikeouts (31 percent). He also throws strikes (72 percent) and limits walks (5 percent).
The knock on Booser, though, is he does allow tons of hard contact.
Booser ranks in the 31st percentile in hard-hit rate (41 percent), 25th percentile in exit velocity (90 mph), and 21st percentile in barrel rate (9.5 percent).
Booser’s best pitch is his four-seam fastball.
The 32-year-old has held opposing batters to a .205 average, .341 slug, and .254 wOBA against his fastball, inducing a 33 percent whiff rate and 41 percent strikeout rate despite a 44 percent hard-hit rate.
Among rookie relievers, the lefty is 1st in strike rate, 5th in strikeout-to-walk rate, 14th in FIP and strand rate (71 percent), 15th in ERA, 16th in ERA+, and 17th in innings.
Among all MLB relievers, the Seattle native is 2nd in strike rate, 18th in strikeout to walk rate, and 48th in FIP.
Furthermore, Booser’s chase rate is in the 98th percentile, strikeout rate is in the 89th percentile, walk rate is in the 86th percentile, and xERA and whiff rate are in the 84th percentile. And he’s in the 70th percentile in xBA (.226) and 61st percentile in xSLG (.383).
Watching players with stories like Cam Booser’s finally fulfilling their dreams—well, that’s part of baseball’s greatness.
Red Sox Rookie Relievers Have Good Fastballs but Not Great Breaking Stuff
Embed from Getty ImagesBoth Red Sox rookie relievers’ fastballs are among the best in MLB. Their breaking stuff, however, isn’t great.
Slaten’s cutter has been a bit lucky this season.
His cutter has a .209 average, a .279 slug, and a .224 wOBA against it. It’s generating a 28 percent whiff rate, a 23 percent strikeout rate, and a 42 percent hard-hit rate.
Per Statcast, batters should have a .257 average, .340 slug, and .268 xwOBA against Slaten’s cutter.
Conversely, Booser’s cutter has been a bit unlucky.
The lefty’s cutter is generating a .350 average, .550 slug, and .417 wOBA with a 26 percent whiff rate, 47 percent hard-hit rate, and a measly 5 percent strikeout rate.
Per Statcast, Booser’s cutter should have a .276 average, a .409 slug, and a .330 wOBA against it.
Slaten’s sweeper, however, has been unlucky. It has a .300 average, a .425 slug, and a .318 wOBA against it. It has a 37 percent whiff rate, a 27 percent strikeout rate, and a 30 percent hard-hit rate against it.
Batters should have a .255 average, a .361 slug, and a .313 wOBA against his cutter.
Booser’s sweeper is okay, but like everything else, it gets hit hard.
The lefty’s sweeper has a .226 average (.236 xBA), .355 slug (.408 xSLG), and .244 wOBA (.270 xwOBA) against it. And it’s generating a 28 percent whiff rate, a 32 percent strikeout rate, and a 33 percent hard-hit rate.
Thankfully, the above hasn’t hurt either of them very much.
With the emergence of rookie relievers Cam Booser and Justin Slaten, among others, the Red Sox bullpen is in great shape heading into 2025.
Featured Image Credit: Maddie Meyer/Getty Images
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