Jonathan Holder came back to the Yankees from Triple-A Scranton/Wilkes-Barre on April 21 hoping his new pitch would work, but ultimately unsure.

The starter-turned-reliever had never thrown a slider before, but after a rocky start to the season in pinstripes, he went to the minors and began tinkering with one. Though Holder had always thrown a curveball, the way batters were hitting him told him something needed to change.

Now they’re telling him the slider was a wise choice.

Holder entered Tuesday without having allowed an earned run in 20 appearances since rejoining the Yankees, becoming an important weapon out of the bullpen and earning manager Aaron Boone’s trust in high-leverage situations.

“I just pay attention to what the hitters tell me,” Holder said before the Yankees kicked off a three-game series against the Mariners in The Bronx. “I see swings and see takes and it’s kind of led me to throw my slider a little bit more.”

Boone and catcher Austin Romine both called the new pitch a “hybrid” between a curveball and slider.

Holder developed it on the fly at Scranton/Wilkes-Barre and called it still a work in progress, but adding it to his arsenal has come with the shutdown results to back it up.

In his first three games of the season before being optioned to Triple-A, Holder was tagged for six earned runs on seven hits across 2 ²/₃ innings. In 22 innings since then, he has allowed just nine hits, three walks and three unearned runs while striking out 21. Opposing batters have slashed .122/.154/.176 during that stretch, making Holder’s emergence reminiscent to that of Chad Green’s 2017, another former starter who found dominance in the bullpen.

Holder’s latest feat was entering Monday’s game against the Nationals with no outs and runners on the corners in the sixth inning, trying to protect a 3-2 lead. He got out of it unscathed.

“He comes in and there’s a fearlessness about the way Jonathan goes about things and his ultimate ability to pound the strike zone,” Boone said. “Now you get the bump in stuff and now you’re getting the guy that’s continuing to have success, continuing to have success in bigger spots and higher-leverage spots, and all of a sudden now we’re seeing a guy that’s become a really important part of our pen and is walking out on that mound with a lot of confidence.”

This season, Holder has thrown just 19 curveballs — nine since he came back from Triple-A, making it his least-used pitch, according to Brooks Baseball. He has gone to his slider 69 times, which trails only his four-seam fastball (192) and changeup (76) — a pitch he always had but has leaned on more heavily this season.

Across his first two seasons in the big leagues, Holder had thrown nearly two-thirds of his pitches (802 total) as curveballs (217) or cutters (266). He threw just 12 changeups and no sliders in that stretch.

“This is definitely a different pitch,” Holder said of his slider. “I’ve just tried to manufacture something that the hitters — maybe it played off my fastball a little bit better.”

While Boone noted that Holder also came back to the majors with a slight uptick in fastball velocity — averaging about 2 mph faster than his first stint this year, per Brooks Baseball — Romine said the velocity of his new pitch has helped its effectiveness.

“He’s throwing it harder, it’s harder for guys to pick up,” Romine said. “The curveball was a little slower, probably high 70s. The slider is more mid-to-lower 80s. Off his fastball, it’s probably harder to see. The deception is a little better and he’s convicted in it. When you’re convicted in a pitch, good things tend to happen.”