Unexpected Red Sox reliever emerging as key high-leverage option

The Boston Red Sox entered this week's series against the Toronto Blue Jays riding high after taking two of three games against both the Philadelphia Phillies and MLB-leading New York Yankees.

Boston's bullpen was key to both of its wins, and it's performed well for most of the season. Brennan Bernardino has been integral to the 'pen's success after he missed the cut for the Opening Day roster. There's another reliever stunning batters since his call-up to the big leagues. That man is Zack Kelly.

Kelly continued his run of success this season in the series finale against the Yankees. After Bernardino loaded the bases and couldn't get any outs in the seventh inning, Alex Cora brought in Kelly to put out the fire.

He did it in grand fashion. He fanned the first two batters he faced and induced a lazy flyout to leave the bases loaded and keep the Yankees off the scoreboard. Kelly's outing was the turning point in the 9-3 win over the Yankees, and the nail in their coffin.

The no-out, bases-loaded jam was Kelly's biggest moment of the season, by far. His body of work this season gave Cora the trust to put him in the game in that spot. Kelly has become the newest high-leverage arm in the Red Sox bullpen, which was necessary with Chris Martin on the injured list.

Zack Kelly has become another underrated star of the Red Sox bullpen

In 17 appearances, Kelly's logged a strong 1.88 ERA and an even 1.00 WHIP, having given up just two home runs and striking out 26 hitters in 24 innings pitched. He's allowed 13 walks this season, which has been his only issue.

Kelly ranks in the 86th percentile in pitching run value according to Baseball Savant, and it only gets better from there. He limits hard contact well, ranking in the 97th percentile in hard-hit percentage. He also sits in the 94th percentile in expected batting average, 88th percentile in average exit velocity, and 84th percentile in both ground ball percentage and fastball velocity.

The right-hander has also done a good job picking up strikeouts and averages 9.8 strikeouts per nine innings. Additionally, he sits in the 78th percentile in strikeout percentage and 74th percentile in whiff percentage.

Kelly's flexed proven ability to pitch multiple innings, which Cora has prioritized this season. Chase Anderson and Brad Keller can go more than one frame as well, but Kelly has been the most reliable long-relief option in Boston's bullpen.

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