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ALEX SPEIER

Inside the numbers of Craig Kimbrel’s recent struggles

Craig Kimbrel has allowed five runs in 6⅓ innings since the All-Star break.
Craig Kimbrel has allowed five runs in 6⅓ innings since the All-Star break.(Barry Chin/Globe Staff/file)

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What’s going on with Craig Kimbrel?

Though the Red Sox claimed a 10-7, extra-innings win Tuesday night in Toronto, Kimbrel continued a troubling trend with a blown save courtesy of a long homer by Justin Smoak in the bottom of the ninth.

In six games since the All-Star break, the closer has been unusually vulnerable. He has allowed five runs in 6⅓ innings (7.71 ERA) with nearly as many walks (5) as strikeouts (7), while opposing hitters have bashed him for a .292/.433/.583 line.

Overall, Kimbrel has been very good in 2018. His 37.1 percent strikeout rate ranks among the highest by a big league reliever, and his .165 opponents’ batting average is one of the lowest. But in contrast to 2017, he has been merely elite rather than otherworldly. And of late, he’s been downright vulnerable.

So what’s happening?

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From the standpoint of outcomes, there are clear differences with Kimbrel from his spectacular 2017 season to this one. His strikeout rate has dipped about 25 percent (49.6 percent down to 37.1 percent) and his walk rate has more than doubled (from 5.5 percent to 11.3 percent). So, he is missing fewer bats and issuing more free passes. That spiking walk rate means that any hits against him have a better chance of turning into runs.

On top of that, when opponents make contact, they are doing more damage. Kimbrel is getting fewer ground balls this year (a 22 percent decline), and perhaps as a result, he is allowing homers to 33 percent more of the hitters he is facing.

Craig Kimbrel's 2017 vs. 2018, by the numbers
Category 2017 2018
ERA 1.43 2.49
Saves 35 33
Blown Saves 4 4
K% 49.6 37.1
BB% 5.5 11.3
K/BB 9/1 3.3/1
Ground ball % 37.0 28.7
HR% 2.4 3.2
Batting average .140 .165
OBP .202 .265
Slugging .243 .344
In strike zone % 47.4 37.6
Strikes swung at (%) 61.2 67.8
Balls swung at (%) 37.4 34.5
Avg. Fastball velocity (mph) 98.3 96.8
SOURCE: Fangraphs, Baseball Savant

Those outcomes are the product of a qualitative step back in terms of both the power and command of Kimbrel’s fastball/curveball mix. His average fastball velocity has ticked back from 98.3 miles per hour last year to 96.8 m.p.h. — a slight drop that nonetheless translates to precious milliseconds for hitters to attack his pitches.

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Some of that drop reflects the fact that he didn’t have a chance to experience a normal spring training while he and his wife tended to their newborn daughter Lydia as she underwent surgeries for a heart defect. Yet after Kimbrel showed a steady increase in velocity through the first half, the temperature on his heater had dipped a bit since the break.

Moreover, hitters have been able to take a more selective approach against Kimbrel this year, given that there is a 20 percent decline in the pitches he is throwing in the strike zone. Whereas he did a great job of locating his fastball at and just above the top of the zone last year — particularly in the middle of the plate, a location that leads to far more chases of unhittable pitches than fastballs away or in — he has seen more of his offerings drift down toward the center of the plate, where they can be hammered.

Already, Kimbrel has given up more homers (6) and extra-base hits (15) on his fastball this year than he did in all of 2017. On top of that, his curveball has been dropping farther this year than it did last year. (Per BrooksBaseball.net, his curveball has dipped to about 16 inches below the middle of the strike zone compared with about 9 inches in 2017.)

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While hitters haven’t been able to do much with the curveball when it’s been thrown for strikes, Kimbrel isn’t doing as good a job of keeping it in the strike zone or selling it as a strike for opponents to chase, helping to explain the spiking walk rate and the decline in swings-and-misses.

Kimbrel has endured midseason speed bumps before and made the necessary mechanical adjustments. It would be impossible for a pitcher to enjoy his kind of career track record without possessing a superb ability to make the sort of subtle tweaks that serve as a tourniquet to downturns.

For a Red Sox team whose October blueprint relies in no small part on consistent dominance from Kimbrel, there is work to be done.


Alex Speier can be reached at alex.speier@globe.com. Follow him on twitter at @alexspeier.