For a while, it seemed like Chris Sale was going to be an American League Cy Young Award shoo-in. Now, while Sale hasn’t exactly gotten much worse, the favorite might be Corey Kluber, who basically hasn’t allowed a run since coming off the disabled list three and a half months ago. If the winner isn’t Kluber, it’ll be Sale. If the winner isn’t Sale, it’ll be Kluber. I don’t know what it would take for neither to win, short of some weird form of voter collusion. The race is pretty obviously down to two horses.

In here, I’d like to highlight the performance of Luis Severino. Severino has not been as good as Sale, and he has not been as good as Kluber. Yet, in part because of those two pitchers, Severino might not have fully gotten his due, because he’s been the next-best pitcher in his own league. Last season, at 22, Severino was demoted from the Yankees’ starting rotation. Now he’s one of the biggest reasons why the 2017 Yankees have overachieved and nearly locked up a spot in the playoffs. The Yankees dreamed that Severino would one day turn into an ace. As young as Severino still is, it seems those dreams might’ve already come true.

It’s not a hard statistical case to make. Why should Severino be considered an ace? Out of all qualified pitchers in either league, he currently ranks fourth in WAR. He ranks fourth in adjusted FIP, and he ranks fourth in adjusted xFIP. He ranks ninth in adjusted ERA, and that’s the noisiest stat of them all. Strikes, strikeouts, and homer avoidance. Severino does everything you’d want from a No. 1. He’s even averaged over 100 pitches per start, so there’s not a real issue of stamina. The profile is there, after nearly all of a full season.

And what is it that makes Severino so good? That’s also not so hard to explain. No AL starter this year has thrown a harder average fastball. Severino has big-time heat, and he pairs that with what can often look like a wipeout slider. Severino has a power repertoire, and when you think power repertoires, you think strikeouts. The success Severino has had makes sense.

But then, he threw hard last year, too. And he had that slider. He lost his starting job, and he finished with an ERA close to 6. There’s still something more to this story. And it seems to be the changeup. Severino, before, struggled to make his changeup do what he wanted, and he spent much of last year as a two-pitch pitcher. In 2017, a better, more consistent changeup has been unlocked, making everything play better. On paper, Severino has improved one pitch. In effect, he’s improved all three.

Let me start with some changeup run values:

2015: -0.6 runs above average

-0.6 runs above average 2016: -0.6

-0.6 2017: +8.5

Run values can’t tell you everything on their own, but they make for a useful general indicator. Severino hasn’t had the most valuable changeup around, but he ranks 12th, and his changeup has roughly the same value as Stephen Strasburg’s. Those numbers are a pretty clear sign that Severino’s changeup has improved, which might make you ask: why? How? Part of it is probably that Severino has increased the gap between his change and his fastball.

Severino’s changeup used to be about seven ticks slower than his fastball, on average. That gap was a little more narrow than the league mean. This year, the average velocity gap is at 10.3 miles per hour, which is one of the larger gaps in the game. The whole idea of a changeup is to be a change from the fastball. There have been pitchers like Zack Greinke and Felix Hernandez who’ve thrived with high changeup velocities, but, generally speaking, more whiffs follow a greater speed difference. Severino has slowed himself down, even while his fastball has played up.

Other pitch traits have also changed. Severino’s changeup now more closely mirrors the movement on his fastball. The difference in horizontal break has dropped from 4.5 to 1.9 inches. The difference in vertical break has dropped from 3.8 to 2.9 inches. That’s something; by spin and movement, the pitches are more difficult to tell apart. But there’s one more change, and it’s the big and obvious one. No pitch is any good if the pitcher can’t control it. And, similarly, a pitcher with control can squeeze value out of even a mediocrity. Severino now is using his changeup differently. He’s pitching with it more aggressively, and he’s spotting it where he wants.

Here are some Baseball Savant heat maps. On the left, changeups from the previous two years. On the right, changeups from 2017. These are viewed from the catcher’s perspective.

I should hardly need to point out what’s different. On the left, fewer than 30% of the changeups are in the zone. On the right, the rate is nearly half. So the changeups on the left generated strikes barely half the time. Two-thirds of Severino’s changeups in 2017 have been strikes. This is a massive change, and it speaks to Severino’s confidence. He’s not just using this changeup as a putaway pitch. It’s something he’s willing to use in any count, and he’s not afraid to put it over the plate. Previously, 31% of Severino’s changeups came in two-strike counts. This year, that rate is just 21%. Severino is using it to battle, and his overall numbers suggest it’s been going quite well for him.

Here’s a 1-and-0 changeup that got a whiff from Pedro Alvarez.

More remarkably, here’s a 1-and-1 changeup that got an ugly swing from Manny Machado.

Given how Severino has used this change, a telling statistic to look up is K-BB% after falling behind 1-and-0. This is how Severino has progressed.

2015: 2.1%

2.1% 2016: 5.5%

5.5% 2017: 17.9%

Here’s this year’s top five among starters in that measure.

Corey Kluber Chris Sale Clayton Kershaw Luis Severino Max Scherzer

Severino is tough when he gets ahead. Tougher than ever. But, he’s always been tough when he’s had the advantage. He’s now better able to work counts into his favor, even counts that earlier might’ve gotten away from him. When he didn’t have a very useful changeup, hitters could try to eliminate it and sit on either the fastball or slider. Now, it’s harder to sit, because in any count, Severino has increased the size of his repertoire by 50%. Hitters can’t eliminate the changeup anymore, because it’s a weapon, but they still have to look for the heater, on account of how fast it gets up on them. By improving his changeup, Severino has taken some pressure off his other two pitches. It’s a classic case of how one specific improvement can lead to improvements in everything else. I don’t think Severino’s slider is much better than it was. Hitters just have more they have to think about.

It’s all so simple, it’s all so classic. A power arm got to the majors with a fastball and a slider. Needed something offspeed to reach the next level. Luis Severino has developed that third pitch, and now all he needs is to have a longer track record. And that’s not the kind of thing you can rush. Aces are crowned after they’ve proven themselves. Severino is now well on his way.