After years of success in finding high-leverage relief options to fill out the backend of the bullpen, the Cubs weren’t able to repeat it in 2023. One veteran addition, Michael Fulmer, has failed to hold onto his late-inning role. The other, Brad Boxberger, was ineffective before hitting the 15-day injured list on May 15 with a right forearm strain. So, Cubs manager David Ross has had to find other in-house options to take on those roles. And who has stood out the most? None other than former-starting-pitching-prospect-turned-reliever Adbert Alzolay.

Alzolay debuted for the Cubs back in 2019 with goals of joining the rotation full-time, but some ups and downs in that role (plus some injuries) forced him into the bullpen. And so far this season, he’s thrived.

Heading into Monday’s off day, among qualified National League relievers, Alzolay ranked eighth in fWAR (0.8), 15th in ERA (2.10), 10th in FIP (2.54), fourth in walk rate (3.6 percent) and sixth in WHIP (0.83). He hasn’t been the best reliever in baseball, per se, but he’s been the rock of the Cubs’ bullpen. While many of the Cubs’ relievers have disappointed, Alzolay has earned Ross’ trust in the big moments.

For someone who’d always wanted to be a starter, it seems he’s found the role that suits him best.

“From being around him, his mentality does seem to suit the bullpen better and seems like a guy that can really rise to the occasion in short bursts,” Cubs assistant pitching coach Daniel Moskos recently told CHGO. “It’s probably hard for him to maintain that through a five- or six- or seven-inning start. So I think you’re starting to see him find his niche, to settle into what his role probably truly should be.”

Beyond the fact he may be a better fit as a reliever, part of the reason Alzolay wasn’t able to find consistent success as a starter — and previously couldn’t be relied on completely in specific pockets — was because he struggled against left-handed batters.

In his last complete major league season in 2021, he had a .246 wOBA against right-handed hitters, but that rose to .384 against lefties. This season, Alzolay significantly reduced his wOBA against lefties (.198) while maintaining his ability to get righties out (.257 wOBA).

But how has he managed to improve on what was a pretty obvious platoon split?

“That’s definitely been the biggest hurdle to overcome is that platoon split,” Moskos said. “For him, I think identifying the right usage was definitely impactful and important for him. And then really just understanding the plan of attack when you’re going in to face a lefty, because it is somewhat different.”

Moskos mentioned the cutter and four-seam fastball as weapons that have really helped him limit damage from the left side. And then there’s a sharper slider shape that’s been years in the making.

“He’s toyed around with some different grips and tried to capture some different shapes,” Moskos said. “The one I think we’ve settled on, really the one he’s going to predominantly feature, is the one that he’s been throwing so far this year. You’re going to see it be a little bit harder, have probably a little bit more of like a downer shape.”

“Because of the velo and because of the shape of it, it ends up being relatively platoon neutral.”

Let’s watch Alzolay’s higher velo 87-88 mph slider that has “downer shape.”

Alzolay understands the value of his harder slider, but he also recognizes that he has the ability to showcase his slower slider when he thinks it’s necessary. Overall, Alzolay is confident in manipulating his slider how he sees fit. And he believes that’s been part of the success he’s found against lefties this season.

“I feel that I can backdoor it, short it, or I can just throw it on the plate, or I can just make it bigger,” Alzolay said on May 31 following an impressive two-inning save against the Rays the night before. “I feel just making that adjustment and having the ability to manipulate that pitch has allowed me to be better against left-handed hitters.”

Here’s what Alzolay’s backdoor, bigger slider looks like to lefties. After you watch him backdoor Yelich, watch it again and pay attention to Yelich’s front knee. See it buckle? Alzolay’s “bigger” slider seemingly froze Yelich.

Now, let’s step in the batter’s box and try to see what hitters see. The below animated graph is Alzolay’s two slider types modeled by velocity, release point, horizontal break, and vertical break while considering gravitational force. The red dot is his “downwards” 87-88 mph slider that has 1 1/2 inches more drop and three inches less horizontal break than his 82-83 mph slider (blue dot).

Along with the sliders, Alzolay agreed with Moskos that the cutter has helped him improve against lefties, in part because he doesn’t have to rely on other pitches in his repertoire if they aren’t there on a given day: “Just the addition of my cutter, too. It’s just three different breaking balls that I’m using against them, that I don’t even have to think about, like, ‘OK, I need to get my changeup right in this situation.’ If the changup is there, I’m going to throw it, but the day that it’s not there, I’m just going to go with my best weapons.”

Now, let’s step in the batter’s box again. If you’re a lefty batter, you’ll see his cutter (black dot) get on you fast. It has less vertical break, sharper plane and more velocity, topping out over 92 mph.

When he debuted four years ago, Alzolay was a completely different pitcher. He threw primarily four-seamers (on 57.3 percent of his pitches). His slider was thrown under 80 mph with twice as much loopy action as his current version, and a changeup rounded out a limited arsenal.

Today, he is a six-pitch reliever who has three unique breaking pitches at 82, 88 and 92 mph that he’s comfortable mixing in against batters on either side of the plate.

The best part about that for the Cubs is that they can use him in more high-leverage moments without having to worry about the specific pockets. He’s found a lot of success in those situations this year, holding opposing batters to a .204 wOBA (according to FanGraphs’ high-leverage split). With the improvements he’s made against lefties, Ross has more faith to use him in high-leverage spots, regardless of which batters are due up, and Alzolay has delivered.

“As a bullpen guy, you’re looking to be able to go through the tough pocket of the lineup, whether it be all right-handed or right/left/right,” Moksos said. “It used to be where we had to try and find him right-handed pockets, and now, it really feels like we can just deploy him up and down throughout the lineup, which is really exciting.”

Alzolay once hoped to be a fixture of this rotation, and the Cubs hoped so, too. But circumstances have led Alzolay into a new role, one where he’s been tasked with getting his team through the high-leverage moments of a game.

May he one day like to be a starter again? It’s possible. But for now, he’s focused on succeeded in the role he’s been given.

“Right now, my main focus is just coming out of the bullpen this year and helping the team,” Alzolay said. “I feel that I adapted to the routine real quick. I feel that I’ve been able to keep my velo, going even back-to-back days or multiple innings. So, I’m kind of enjoying this journey right now out of the bullpen. That’s the only thing I have on my mind.”