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What’s wrong with Corey Knebel?

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There are plenty of theories why the former All-Star closer is struggling to repeat the success of his 2017 season, but it all stems from his problems with one particular pitch.

MLB: San Diego Padres at Milwaukee Brewers Jeffrey Becker-USA TODAY Sports

This is going to sound crazy after Corey Knebel’s most appearance on Thursday afternoon, where he walked the bases loaded full of Padres — the team with a collective OBP under .300 — but it’s true: walks are not Corey Knebel’s problem this year.

Yes, he still walks too many to be a truly comfortable 9th inning option, but the same was true last year. And, believe it or not, he’s actually walking batters at a lower rate than he did last year. Even after Thursday’s debacle, Knebel’s BB/9 and BB% (4.54 and 12.1%) are an improvement over his 2017 marks (4.74 and 12.9%). He’s even significantly improved his groundball rate from 38.3% to 49.4%.

So why is his ERA 3 full runs higher than it was last year?

The problem isn’t necessarily his curveball, as has been cited on numerous broadcasts as he’s struggled. It could actually be his fastball.

It seems a little weird to say about a guy who’s still carrying a 12.62 K/9 and striking out a third of the guys he’s facing, but Knebel isn’t missing bats like he used to. Batters are making contact more frequently against him, and when they do, the ball is traveling far.

While he’s not allowing as many fly balls, the ones he is giving up are flying out of the park at a much higher (and frankly, probably unsustainable) rate. After two years of HR/FB rates in the 9% range (9.1% in 2016, 9.5% in 2017), it’s up to 30.4% this year. As hittable as he’s been this year, that’s still likely to come down at some point. That mark is the third-highest in the league right now, and his stuff is too good for him to stay in the same company as Dan Otero and Hector Neris.

Still, it’s a sign guys aren’t getting fooled right now. Not surprisingly with home run numbers like that, his hard hit rate has also spiked from 32.6% last year to 45.6% this year. A big part of that has been opponents teeing off against Knebel’s fastball, which has gone from one of the best in the league to a mediocre pitch this year.

A lot has been made about Knebel’s seeming reluctance to throw his curveball this year, and it makes for a convenient explanation for all of the hard-hit stats. But while he has used the curveball less this year, it’s not like he’s completely abandoned it — his curveball usage is down about 2% compared to last year overall, but that difference is small. According to Brooks Baseball, Knebel has thrown 173 curveballs through 35.2 innings this year. Through the same amount of innings last year, Knebel had thrown 185 curves — just 12 more.

Maybe that was enough to keep hitters just late enough on his fastball, but that’s not even one additional curveball per inning. What seems more likely is that fewer batters have stopped chasing Knebel out of the zone, whether he’s throwing a curveball or a fastball, and the ball isn’t missing bats inside the zone.

The numbers bear that out. His opponent O-Swing% — or the percentage of swings he’s drawn on pitches outside of the strike zone — has dropped by 3.5% this year, from a career-best 28.8% last year to 25.3% this year. His Z-Swing% — or percentage of pitches swung at in the strike zone — has spiked from 61.9% to 68.1%. On those swings in the strike zone, opponents have upped their contact rate from 74.7% to 79.8%.

Basically, batters are making contact on 4 out of every 5 pitches Knebel does throw in the strike zone, and he’s throwing more pitches in the strike zone (51.8%) than he ever has in his big league career. It likely doesn’t help that Knebel’s fastball is coming in about a half-tick slower than it was last year, either (96.8 mph compared to 97.4), and it’s moving less than it did last year, too.

Despite having less movement, Knebel seems to be having trouble putting the pitch where he wants it. Here’s a look at where his fastball location has been this year vs. last year, courtesy of Fangraphs’ heatmaps (for context, these are from the pitchers’ point of view, so right-handed batters would stand on the right, lefties to the left):

Courtesy: Fangraphs
Courtesy: Fangraphs

That’s a lot of straight middle-middle fastballs, and he hasn’t been able to find the edges of the strike zone, so it’s really no wonder the hard-hit rates have spiked. You can also see a pretty clear effort on Knebel’s part to get guys to chase the high fastball this year, but the majority of those pitches aren’t close enough to get them to bite (again, going back to the lack of swings outside of the zone).

For all the talk about his curveball — and yes, he could stand to throw that pitch for strikes a little more often — he also needs to figure out what’s going on with his fastball that’s caused it to drop from 11.7 runs above average last year to 2.2 runs below average this year, ranking it among the worst 30 fastballs among MLB relievers.

It’s hard enough to find success against Major League hitters when you’re fighting your command on off-speed pitches. It’s nearly impossible to get them out when you’re throwing batting practice (albeit high-speed batting practice) pitches.

Statistics courtesy of Fangraphs