What's ailing Severino? Start with his slider

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Through the All-Star break, Luis Severino was legitimately one of the best pitchers in baseball. His 2.31 ERA was seventh-best among Major League starters in the first half, and his 2.65 ERA dating back to the start of 2017 was fourth-best, behind only a quartet of potential Hall of Famers in Clayton Kershaw, Corey Kluber, Max Scherzer and Chris Sale. He finished third in the AL Cy Young voting last year, at just 23 years old. He was so, so good.

Over his last seven starts, Severino has allowed 30 earned runs in 36 innings, which is a 7.50 ERA. That's not quite the worst mark of any regular starter in that time -- we see you, Burch Smith's 9.67, Luis Perdomo's 9.00, Jon Lester's 8.65 ERA, and a handful of others -- but it's in the neighborhood.

The Yankees already had a rotation problem, thanks to Sonny Gray's struggles, Jordan Montgomery's elbow, and now CC Sabathia's knee, which is why they had to go get J.A. Happ and Lance Lynn. Now, they've got an even bigger problem, as their 2018 goal has become less "catch the Red Sox" and more "hold off the A's and Mariners." What in the world is wrong with their ace? Is this a poorly timed slump or the sign of something more serious? Let's find out.

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The Yankees and Severino themselves insist it's not about health, or fatigue. "I'm not tired. ... It's nothing out of the ordinary," Severino said after allowing allowing four runs in five innings to the Mets on Monday. "Physically, I think he's sound," added manager Aaron Boone.

That's more or less what you'd expect them to say, but the first place you'd look to try to find a potential health issue would be fastball velocity, and it seems there's not much to worry about there.

First 18 starts: 97.7 mph
Last seven starts: 97.3 mph

Even on Monday night, Severino hit 99 mph four times. It doesn't seem like velocity is the problem.

That said, Severino's fastball has been absolutely crushed during the span in question.

First 18 starts: .214 average / .318 slugging / 23 percent swing-and-miss rate
Last seven starts: .434 average / .803 slugging / 13 percent swing-and miss rate

That .318 slugging? It was the fourth-best of regular starters through July 6. That .803 slugging? It's very nearly the worst, and the only starter hit harder than Severino, Kenta Maeda, has already been sent to the bullpen.

That's a stunning difference, but there's not much new about the fastball that stands out. The velocity is the same. The spin hasn't changed. He's using it the same amount, 50 percent. He's throwing the same amount in the zone, about 60 percent. Those are the first things you'd look at to explain such a change, but there's nothing here. So what's going on?

Instead, allow us to put forth a theory: Severino's slider has been much less effective, and because of that, his fastball has been less effective, too.

For all of the heat -- last year, it had the highest average velocity of any regular starter -- Severino's fastball has always been relatively straight. (It had a very similar profile to Shohei Ohtani's fast-but-straight fastball when we profiled Ohtani's Japan data last winter.)

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Obviously, it's still been a very effective pitch, but Major League hitters can handle high-90s heat when it doesn't have a ton of movement, unless they're thrown thrown off balance by anticipating an elite breaking pitch. For Severino, that pitch is the slider, which also had elite velocity last year (88.3 mph, top 10 among regular starters), and elite performance. In 2017, by one measure, it was the sixth most valuable slider.

For most of this year, it was very good, too. But like his fastball, the slider has also been getting hit a lot harder lately, though the swing-and-miss has been unchanged.

First 18 starts: .152 average / .217 slugging / 38 percent swing-and-miss rate
Last seven starts: .271 average / .508 slugging / 37 percent swing-and miss rate

It's the same story: that .217 slugging was a top-five mark among regular starters, and the .508 mark is near the bottom.

The difference here is that unlike with Severino's fastball, there have been clear changes -- and probably not for the better -- on his slider as the season has gone on. The movement is different. The spin is different. It's just ... different.

Through Severino's first 18 starts, he had the highest spin rate in baseball on his slider, among starting pitchers. Because it started so high, it's still well above-average, but it's down about seven percent in August from where it was before that, and his last two starts have had two of his three lowest spin rate numbers.

Slider spin is a tricky thing. With four-seam fastballs and curveballs, you can made broad assumptions from the raw spin rate numbers (for four-seamers, for example, high spin lends itself to swinging strikes and flyballs, while low spin leans toward sink and grounders). But slider spin doesn't quite work that way. High spin doesn't correlate directly to good, though it's interesting to at least note that anecdotally, the names at the top of the slider spin list are more impressive than the names at the bottom.

By itself, the slider spin change wouldn't be that noticeable, except for the fact that there are some pretty clear movement changes happening at the same time. In April, Severino got nearly 11 inches of horizontal movement on his slider, and it hovered around 10 inches for the next three months. In August, that's down to only nine inches of movement, and yes, two inches is a pretty big deal in baseball terms.

The same goes for the amount of drop. Severino's slider dropped nearly 40 inches (this includes gravity, unlike other movement numbers you may have seen) in April. In August, it's down below 37 inches. The raw numbers here don't matter so much as the takeaway: Compared to April, Severino is getting three fewer inches of drop and two fewer inches of break on his slider.

Remember, this is how that pitch is supposed to look, dropping and fading. Now imagine it with three fewer inches of drop and two fewer inches of break. It's not the same.

That's why he's getting fewer misses outside the zone on it (73 percent chase swing and miss in April and May, 63 percent since). It might be why he's getting fewer grounders on it (66 percent in April, down each month to 41 percent in August). It might be why he's simply throwing it in the zone less often (58 percent of the time in June, 37 percent in August), if he's worried about leaving a hanging pitch to get hammered.

You can see how that would travel back to the fastball, too. If you don't have to worry so much about getting fooled by the wipeout slider, you can sit on the fastball, and it's a lot easier to time a 97-mph four-seamer if you aren't thinking about a secondary pitch as much, and if it's coming in straight. Everything here is connected.

Now, why exactly that's happening is a little less clear. It could be something simply mechanical; as Severino said after the Mets game, "I keep making the same mistakes, over and over." It helps to refute the idea that he's tipping his pitches, because tipping wouldn't make pitches move differently.

The answer here isn't that Severino is injured or broken or unfixable. It might be as simple as one of those tiny little tweaks and adjustments, often imperceptible, that pitchers are constantly making as the season goes on. He's simply too talented not to perform better than he has over the last few weeks. This might just be a slump.

It's not nothing, though. The damage we're seeing here is real -- and Severino and the Yankees need to figure out how to fix it. After all, an AL Wild Card Game at home isn't guaranteed. Neither is the Wild Card Game at all. The Yankees won't get there without their ace.

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