There’s a sense that Jake Arrieta isn’t quite what he’s been before. It’s not entirely untrue — a season ago, Arrieta put together a historic campaign. He set the bar so high for himself that it would be next to impossible to meet the updated expectations. But, you know, Arrieta’s still been terrific. Last year, opponents batted .185. This year they’ve batted .183. He ranks fourth among qualified starting pitchers in ERA, and even since his ERA dipped under 1 around the beginning of May, it’s been just a little over 3. Last year’s National League Cy Young came down to Arrieta, Zack Greinke, and Clayton Kershaw. Kershaw’s hurt. Greinke’s already allowed 19 more runs than he did a season ago. Arrieta is doing just fine.

He simply seems a wee bit less automatic. From the perspective of an observer, he’s made it more difficult to take outs for granted. From the perspective of an analyst, Arrieta’s command has wobbled. And what’s maybe most interesting here: Arrieta apparently doesn’t have a feel for his slider. Or cutter. Or whatever. You know what I mean. Arrieta has managed a low ERA while, underneath, he’s had trouble finding what had been one of the truly elite pitches in the game.

For all the elements that went into Arrieta’s maturation and emergence, one of the very biggest keys was his unlocking his slider. They hadn’t let him use it too much in Baltimore, but in Chicago, he came to embrace it. For so many reasons, it’s not an easy thing to objectively rate individual pitches. But we can get a lot of the way there just making use of our pitch-type run values. So: Between 2014 – 2015, Arrieta’s slider was worth about 39 runs above average. The only pitches with higher values were Clayton Kershaw’s fastball, Johnny Cueto’s fastball, and Tyson Ross‘ slider. It’s not perfect, but you can agree with it. All of those pitches were great. They were thrown with great frequency.

So when you look at Arrieta now, his nerd numbers are jarring. For the season, his slider has a negative run value. And he’s thrown the slider far less often, picking up the use of his heater. This plot traces the course of things. Included below are Arrieta’s rolling slider rates, and slider values, over 15-game spans.

It’s very easy to spot when Arrieta arrived in Chicago and got comfortable. He folded the slider in heavily, and the pitch itself improved, which is reflected by the sharp increase in the blue line. For the most part, the two lines hold pretty steady. And then there’s the drop-off. The reduction in usage has picked up. The run value has plummeted. Between 2014 – 2015, Arrieta had 23 starts in which his slider was at least a run better than average. This year, he’s had three such starts, the last one coming on May 3. I don’t know that there’s anything special about that particular date, but the slider hasn’t looked like itself since. And the usage, in turn, has come down.

It’s a fascinating change in Arrieta’s profile. And it’s not an easy thing to figure out. The slider is a little slower than it was a year ago, but it’s right where it was in 2014. And, by movement, the shape hasn’t changed. It looks like the same slider, but hitters are seeing it a little better, laying off of some tough pitches. It’s not hard to connect the taken sliders with Arrieta’s increase in walks. If I had to speculate, I’d go with this. According to Brooks Baseball, Arrieta’s arm slot has slightly risen:

That’s given him a more vertical angle, or plane, and that might be contributing to Arrieta throwing more sliders below the zone instead of in it or beside it. The relevant slider heat maps:

A few years ago, Arrieta threw 24% of his sliders no more than a foot and a half off the ground. Last year’s rate was 18%. This year he’s up to 32%. Now, usually, pitchers want to bury their sliders, as opposed to hang them, but maybe something about Arrieta’s slider makes it easier to identify when it’s dropping. Maybe it really is best when it’s around the knees, instead of the shins. That’s what the numbers would indicate. If Arrieta still believed in the pitch as much as he used to, you’d figure he’d still throw it. It’s around, but maybe half as often as it used to be. That slider just isn’t the same kind of weapon, after a couple years of hitters having no answer. Either they’ve adjusted to it, or it simply looks different, even though it’s breaking the same.

This is where we go back to the fact that Arrieta has an ERA around the mid-2s. And this is where we talk about why Arrieta isn’t just some flash in the pan. Arrieta didn’t ride one pitch to greatness. Arrieta, more broadly, just figured out pitching, and he figured out how to make the necessary adjustments. He throws, occasionally, a changeup. He throws, more commonly, a curveball. And there’s the fastball. Used to be, by run value, Arrieta’s slider was one of the best pitches in the game. Now, via the current leaderboards, Arrieta’s fastball rates as the top pitch in the game. It’s picked up the slack, and it’s possible the changes that made the slider worse have made the fastball more effective. Not that I think Arrieta wants to be without his best slider, but, he’s compensating. That’s what the best ones do.

One takeaway would be that, in 2016, Jake Arrieta has seemingly lost his signature slider. Another takeaway would be that Jake Arrieta has seemingly lost his signature slider, and still he’s an ace. You’re free to take from this whatever you want.