Andrés Muñoz’s new sinker might be just what he needed

Andrés Muñoz doesn’t have to be better than he was in last year’s second half. Starting on June 13, roughly when he reworked his slider, he struck out 43.8% of the batters he faced, while walking just 5.6% and allowing zero home runs. Thanks to throwing eight more innings than Edwin Díaz, that made him FanGraphs’ most valuable reliever in MLB over that four-month stretch. But his playoff performance, especially against the Astros, sent him into the offseason with a reminder that you can always get better.

But how, exactly, is a pitcher like Andrés Muñoz supposed to get better? After amping up his slider, it became one of MLB’s nastiest pitches, with an otherworldly -3.4 RV/100. That is to say, he would give up 3.4 runs fewer than average for every 100 sliders thrown. A simple way to imagine the impact of that is that the difference between Muñoz’s slider and the average MLB pitch is roughly the difference between starting Luis Castillo and starting Justus Sheffield. His four-seamer wasn’t as good, but there’s a limit to how much you can improve a pitch that averaged 100.2 miles per hour, magnified by one of the lowest vertical approach angles in the bigs.

But one thing you can do is to add a new pitch. And that’s how Andrés Muñoz came to throw a sinker. His first four outings of 2023 have me pretty psyched about it.

By stuff, it already lines up as a great sinker. No sinker that’s as fast as his moves as much; no sinker that moves that much, comes in as fast. Caveats abound: small samples, pitchers still ramping up velocity, etc., etc., etc. But here’s how his sinker compares to the other pitchers who’ve thrown at least ten this year.

Chart: Zach Mason for Lookout Landing * Source: Baseball Savant

A sinker is the perfect addition for Muñoz, as it should complement both his slider and his four-seamer. It works with the slider because like the slider, it drops as it approaches the plate, but it moves arm side where the slider moves glove side. “I’m trying to get the sinker to drop, like how my slider drops,” he says. “I want to get the same thing with my sinker, so it goes straight and then at the end moves down and away.” Here’s what it can look like to get the two pitches back to back. First the slider:

Now watch the sinker on the next pitch. Ignore for the moment how far outside the zone it gets (we’ll get back to that), and focus on the way it looks like you held a mirror up to the slider.

Add in the fact that the sinker comes in about 10 mph faster, and imagine trying to get your barrel on a ball against Muñoz. Think you can pick up the difference from the spin? Maybe, but I doubt it. They don’t spin in exactly the opposite directions. Ideally, you’d want the sinker to spin at 4:15 to line up with the slider’s 10:15. But 2:45 is close enough to make them hard to distinguish.

By themselves, Muñoz’s sinker and slider could turn into a lethal combination. Worse versions of those two pitches was enough for Diego Castillo to be the set-up man for good teams. But despite the fact that Muñoz has only thrown three four-seamers out of his 49 pitches this year, the plan is not to replace it with the sinker. He says he’s only focusing on the sinker early to try to round the pitch into shape and figure it out against live competition.

“I didn’t have a normal Spring Training,” he notes, referring to his late start necessitated by his recovery from the foot surgery he had during the offseason. “I only had three outings over there, so I didn’t have a chance to try [the sinker] how I wanted it,” he said. “I just want to know how they look.”

I’m on board with the plan to keep the four-seamer. Having two different fastballs that come in at roughly the same (elite) speed should cause both to perform better. Thanks to the way he falls off the mound, it seems like his four-seamer doesn’t get the perceived rise you’d expect based on the velocity and VAA. But if he’s got a different fastball that actually drops, that may help make the four-seamer appear to rise a bit more. It’s hard to identify that you’re getting a fastball, and even harder to time up one that comes in around 100 mph. But now, even if you can do both of those things, it still won’t be enough to barrel it up since there’s two to think about and they move differently.

Based on the stuff, the sinker is already a top-shelf pitch; what’s holding it back is command, which may or may not come with time. If you’ve been browsing his Savant page, you might have noticed that his sliders are coming in higher in the zone than his sinker. But that’s not some avant-grade strategy. Most of the time, he hasn’t even hit the same quadrant as Cal’s target.

Grid View

It’s an enviable problem. “The way it’s moving so much, it’s hard to control,” he says. “Even if that thing starts in the middle, it can finish as a ball.”

The one that came closest to where Cal called for it got one of his two swinging strikes on the pitch.

But whether he’s ever able to paint with it or not, I’m confident that his command will improve from where it is now. Despite the early trouble, he’s clearly working on it, and he should get better as he throws it against live hitters more often. What’s more, the Mariners pitching department has told him it’s more natural for his arm slot, so he thinks he’s able to control it better, even if that hasn’t shown up in games yet.

So far, it’s mostly working anyway, either because hitters haven’t had the benefit of advanced scouting on the pitch yet or simply because it moves so much and so fast. He doesn’t have the precision with it to get whiffs yet, but it’s still doing what sinkers are best known for: getting weak contact. Of the six times hitters have made contact, nobody’s topped an 81-mph exit velocity, and all six were smashed into the ground. As Muñoz put it, “The good thing is I can get a lot of ground balls with that pitch, because it’s going below the bat, so that creates a lot of ground balls. Double plays, that’s the best.”

Unfortunately, Muñoz has been on the IL since April 8 due to a strained right deltoid, something he attributes partially to the shock of going from a shortened spring training in warm Arizona to the sudden cold of Seattle and Cleveland in April. But the IL stint has been precautionary; he never stopped throwing, and he’s about to start a rehab stint with the Rainiers in Las Vegas. That’ll give him even more chances to refine the sinker against live competition, and it could be scary for hitters once he gets back, hopefully next week. Last year, Andrés Muñoz was a top-five reliever, and that was with a fastball that underperformed its metrics. What’s he going to look like if he’s got a good fastball too?