Any Cubs fan could tell you Jake Arrieta just doesn’t seem quite right. This has been the case for a number of weeks, or months, and we’ve touched on it before. It’s funny to think about the hand-wringing over a pitcher sitting on a sub-3 ERA. But, he really has gotten meaningfully worse, and he’s the staff ace of a World Series favorite. Cubs fans don’t have a whole lot of negatives right now. Arrieta might be turning into one, and he had a clunker against Pittsburgh just Monday.

Last week, I wrote about the curious disappearance of Arrieta’s unbelievable slider. It hasn’t killed him or anything, with the fastball picking up the slack, but that’s not something Arrieta would’ve wished for. The slider going away is a symptom of something. I can present to you now another symptom of something. And it’s related to the slider’s deterioration — Arrieta’s developing a platoon split. He’s having some trouble with lefties, and, it turns out there are lefties everywhere.

What an excellent opportunity this is to make use of our new splits tool. To get right to it, here’s Arrieta’s entire big-league career, broken down by wOBA allowed to lefties over 10-game stretches.

You can see something there. It’s also not real dramatic — Arrieta has been worse of late, but this still makes him look plenty effective. Part of the problem here is that wOBA allowed doesn’t adjust for the Cubs’ defense, and that defensive unit has yielded a collective BABIP of like .079. So, as is usual, it makes sense to dig deeper, into the peripherals. For example, how has Arrieta been doing with regard to simply throwing strikes to lefties?

There’s a steep cliff, back toward a baseline. Arrieta built and built and built, and then something got abruptly worse. This looks even worse in the next plot, showing K-BB%:

Here it’s easy to see when Arrieta became a Cub. And, for a little while now, he’s been slipping. Lefties have been walking more and more, without a strikeout increase to compensate. Over the last stretch, Arrieta’s down in the single digits. And finally, as the rolling-average plots go, here’s hard-hit rate. Arrieta isn’t even necessarily suppressing quality contact.

Hard-hit rate, naturally, fluctuates. And overall, Arrieta still isn’t getting killed or anything. But to borrow from Baseball Savant, and Statcast — last season, facing lefties, Arrieta ranked in the 83rd percentile in lowest average launch angle, and he ranked in the 85th percentile in lowest average exit velocity. This season, his percentile rankings are 50th and 59th, respectively. In other words, they’re unremarkable. Lefties have been better able to lift the ball, and they’ve been better able to drive the ball. That’s bad, especially when combined with the changes in strikeouts and walks.

It’s pretty clear to see how Arrieta is locating differently. Again, this is just against lefties. Observers have insisted for a while that it feels like Arrieta just doesn’t have the same command as he used to. That can’t really be argued.

This is from the catcher’s perspective, so what you’re seeing is a reduced ability for Arrieta to hang around the outside edge. You see him more over the middle of the plate, and slightly more down. Arrieta doesn’t want to be more over the middle of the plate, so this reflects a problem with location. It’s not hard to see how that could be tied to a decline in the success of his slider — something is presumably up with his delivery. It’s costing him command, and it’s costing him what used to be probably his best pitch. Now it’s easier for lefties to take pitches out of the zone, and to hit pitches over the plate. There are fewer pitches in that gray area away, and they haven’t had to worry nearly so much about quality sliders under the hands.

I want now to show you a few pitches. First, a representative pitch thrown with the bases empty by 2015 Arrieta.

Now, a representative pitch thrown with the bases empty by 2016 Arrieta.

The difference in result is obvious — the first pitch was located perfectly, and the second one sailed arm-side. It’s not the result I care about so much. It’s what went into the result. A comparison of stills:

This is Arrieta removing the ball from his glove. The pictures are similar; the delivery is similar. Arrieta hasn’t made any giant changes. But I do think something is starting here — Arrieta, on the right, seems to be leaning back. He’s already dropping on his back leg, and it looks to me like his shoulders are tilted. Fast-forward a few frames:

Here, there are two things. One, on the right, Arrieta appears more over-rotated. More of his chest is facing the center-field camera; also, you can see more of the logo. Arrieta has exaggerated his shoulder turn. And then you can see the difference in his left leg. On the left, Arrieta is calmly striding forward. On the right, the stride seems more aggressive, and Arrieta has his heel higher in the air, off the dirt. His timing is changed, given how much that all depends on when he plants his foot, and there’s also a different energy transfer that, just visually, seems more aggressive than it was. Last year’s Arrieta smoothly shifted from back leg to front. This year’s Arrieta looks more like he’s reaching back for something. It could be just my own perception, but clearly something is off. And Arrieta might be more forcefully shifting to his front leg, just as his front leg is coming down with different timing.

I won’t pretend like I have all the answers, but I also won’t pretend like I don’t see anything. We know something must be off with Arrieta’s delivery, and I’m seeing some differences right up there. Maybe Arrieta just feels like he has to throw more forcefully to get that velocity he wants. Maybe things have just gone slightly awry naturally, and all he needs is a few bullpen sessions. Arrieta knows he needs to do better, the Cubs know he needs to do better, and everyone knows the Cubs have the present advantage of low stakes. There’s time to work this out. Yet if the struggles keep up, suddenly the Cubs don’t seem quite so invincible. I guess the other contenders need to hold on to something.