LOS ANGELES — Zack Wheeler was already having a strong season when he met last month with pitching coach Caleb Cotham, who wanted to know what else the Phillies’ pitcher wanted to do.

Wheeler signed with the Phillies before last season as a hard-throwing strikeout pitcher but showed last summer that he can have success by inducing weak contact. He was excellent in 2020 (2.92 ERA in 11 starts), and after a quick start to 2021 it would have been easy for Wheeler to keep pitching the same way.

Instead he told Cotham, the team’s first-year pitching coach, that he wanted to improve his slider. It was a good pitch, Wheeler said, but it was a pitch that a hitter had to chase in order for it to be effective as he rarely landed it in the strike zone.

In the midst of an already good season, Wheeler wanted to add a new wrinkle.

“In a lot of ways, he could be one of the better guys in the game with just his fastball. And he is,” Cotham said. “But it does speak to how good he wants to be and how much attention to detail he has and the things he thinks about when he goes home like ‘How can I be better?’ But also at the same time, not sacrificing who he is. It’s taking who he is and nudging it.”

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Wheeler enters Wednesday’s start at Dodger Stadium with a 1.39 ERA in his last seven starts and has thrown the second-highest innings total this season among all pitchers with 90⅓. He leads the National League in strikeouts, has the second-best ERA among all big-league starters, and trails only Jacob deGrom in WAR among pitchers.

The 31-year-old has developed this season into one of baseball’s premier pitchers and is on track for his first All Star nod. And a big part of his development has been the improvement of his slider. Or is it his cutter?

“A lot of people are confusing my slider with my cutter,” Wheeler said.

The cutter and the slider are already similar pitches, and Wheeler’s slider moves so hard that pitch-tracking websites become confused between the two. MLB’s Statcast data does not list a cutter among the 1,321 pitches Wheeler has thrown this season.

But he insists the pitch has played a big part of a defining season.

“I would probably say if I throw ’20 sliders,’ five are actually sliders,” Wheeler said.

Wheeler throws the cutter up and in to left-handed hitters while throwing his slider down and away to right-handers. The pitches have been so effective because they’re now being thrown as strikes, often peppering the very corners of the strike zone. It’s no longer possible for a hitter to just lay off Wheeler’s slider or cutter.

Both pitches break from right to left, but the cutter breaks later and sharper with a biting finish. The slider moves across the plate, yet Wheeler’s slider still moves faster than most. Both pitches leave Wheeler’s hand with the same motion and from the arm slot, making it hard for a hitter — and the pitch-tracking systems — to tell the difference.

“I think when he throws it up, it’s going to be flatter. When he throws it down, it’s going to have more depth,” manager Joe Girardi said. “But they measure those things by spin rate, which I never truly understand. One’s a slider, one’s a cutter. Truly, they’re all sliders. He throws a really hard slider.”

Wheeler has thrown the slider/cutter this season for 25.6% of his pitches, a nearly 10% increase from last season according to Statcast. Opponents are hitting just .162 against it after hitting .231 against it last season.

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He’s not only throwing the pitch more, but it’s more effective. The pitch (or pitches) has given Wheeler a new weapon to use.

“Guys with velo need their off-speed,” Wheeler said. “If [batters] can just say ‘No’ right away to the slider because it’s not going to be a strike so I’m not going to swing at it, then it just takes a pitch away from you no matter how good you think it is. When it’s there and has a shape and has consistency and you can throw it with the same arm path and same arm movement, then it’s going to be a good pitch.”

Wheeler struck out nearly nine batters per nine innings in his final two seasons with the Mets prior to joining the Phillies before last season on a $118 million contract. His fastball neared triple digits and it was often overpowering.

But last season, he didn’t shy away from contact and registered the highest ground ball rate of his career (55.9% according to FanGraphs). This season, Wheeler is racking up strikeouts at a career-high rate (11.2 per nine innings) while still maintaining a ground ball rate (47.1%) that ranks 15th among National League starters.

Wheeler’s dominance has been the result of finding a way to do the two things he’s done well in his career — inducing weak contact and striking out batters — at the same time.

“He’s as complete as it comes right now,” Cotham said. “We love strikeouts from the pitching side, but we also love weak contact and we love pitching deep into games. If you’re chasing strikeout right away, it doesn’t typically work. He’s a really good blend of both and he’s doing it at a high level.”

Wheeler lived up to his contract last summer and was carrying it over into this season. But he wanted to do more than just earn his pay. It’s discussions like the one he had about his slider that have morphed him this season into one of baseball’s premier starters.

“You’re always trying to get better and you see guys throw nasty stuff and you’re like ‘I want to throw that’ or ‘I want to have that,’” Wheeler said. “It was a good pitch for me, but I wanted it to get better and get more consistent.”

“I think he has the ability to kind to do whatever he wants to do with the baseball,” Cotham said. “He picked it up and it speaks to the talent he has and the ability in his hand.”