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Has Luis Severino found a way to get even better?

Through two starts, the Yankees’ ace has leaned heavily on his slider, and with good reason.

Earlier this week, I detailed how the Yankees have steered clear of their fastballs early on. The starters have uniformly moved away from their four-seamers and two-seamers, with their secondary pitches taking on a more prominent role. It has only been a week, but this continues a trend, as the Yankees have been among the least fastball-centric teams in the league in recent years.

If there was a pitcher for whom you would think this trend might not hold, it’s Luis Severino. The 23-year-old flame-thrower has one of the hardest four-seamers in the game. In fact, among starters last year, it was the hardest four-seamer in MLB. Severino can comfortably sit at 98 mph and dial up triple digits when he needs to. His heater is the deadliest one in the league this side of Noah Syndergaard.

Yet, through two starts, Severino too has decreased his fastball usage in favor of his best secondary: the slider. After a couple games, the slider has become Severino’s weapon of choice.

That might seem counter-intuitive at first glance. Why would Severino pitch backwards? It makes complete sense when the likes of CC Sabathia and Masahiro Tanaka use sliders and splitters to keep hitters off balance. Sabathia struggles to consistently crack 90 mph now, and Tanaka has long had a middling fastball. It’s no surprise they often use their secondaries as their de facto primary pitches.

Given Severino’s raw power, it would seem strange that he wouldn’t establish his fastball, and then work his secondaries in from there. In 2018 so far, though, Severino has attacked with his slider almost as much as his fastball. Per Brooks Baseball, Severino has used his four-seamer 48% of the time, the lowest rate of his career, and his slider 44% of the time. That’s a substantial jump from his 34% slider rate in 2017.

Severino was hugely successful in 2017, so why change dramatically? Well, despite his four-seamers’ obvious merits, his slider just might be his best pitch. If it is his best offering, then using it more could help Severino stay at the level he attained last year, or even ascend.

According to Brooks Baseball, Severino’s fastball notched whiffs on 23% of swings in 2017, an excellent rate for a heater, but that paled in comparison to the 33% whiff rate his slider earned. Moreover, Severino’s slider generated a grounder on over 63% of balls in play, a startling number. Essentially, when opponents offered at Severino’s slider, they were almost always coming up empty, or hitting nearly harmless grounders.

Unsurprisingly, then, opposing hitters posted only a .181 batting average against the slider, to go along with a microscopic .280 slugging percentage. Simply put, the results Severino generated on the pitch last year were tremendous. Per FanGraphs’ pitch values, Severino’s slider was the most valuable such pitch in all of baseball.

Now, zeroing in on something like pitch values, or batting average on a particular pitch, is a narrow view to take. All pitches work in concert with each other. Just because Severino’s four-seamer generates worse results than the slider doesn’t mean he should discard it; the slider likely posts outstanding numbers in some part thanks to the way it interacts with the fastball.

That being said, Severino’s slider puts up eye-popping numbers, and using it more than he did last year looks like the smart move. The samples to this point are all but meaningless, but through two starts, 12 of Severino’s 14 strikeouts have come courtesy of the slider, and opposing hitters have just two hits on 25 balls in play against the pitch.

Perhaps most interestingly, the slider appears to be moving even more in 2018 than previously, at least according to Brooks Baseball’s pitch-tracking. The pitch has averaged over six inches of horizontal movement in 2018 and over three inches of drop, after averaging about three and one inches in 2017.

It’s possible something wonky has gone on with early-season pitch-tracking, but Severino’s slider does look particularly wicked so far, at least anecdotally. Compare a slider he unleashed on the Blue Jays on opening day:

To one he threw in Boston last season:

If Severino’s slider is even more deadly this year, and he commits to using it as basically his primary pitch, the sky might be the limit. He was already excellent in 2017, rolling to a top-three AL Cy Young finish. In using his best pitch more often in 2018, Severino just may have found a way to improve on that showing. The rest of the American League should be wary.