Orion Kerkering is known for his slider and his mental toughness. Meet the coach who taught him both.

Orion Kerkering is about to do something extraordinary. He’s 22, he’s jumped from low A to the majors in the same year, and he likely will find himself on the Phillies’ postseason roster next week. As of Friday, he’d pitched only two big league innings. It’s a small sample size for a roster spot that consequential, but the Phillies are not concerned.

Manager Rob Thomson has maintained that he saw all he needed to see in Kerkering’s debut on Sept. 24 against the Mets. The right-handed reliever flashed his devastating slider, with 18-20 inches of break, but also his mindset. While he mowed down three big league hitters on just 12 pitches, he looked unfazed. Thomson joked that Kerkering didn’t break a sweat.

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It’s a quality you want in a reliever, and a big reason the Phillies have as much trust in Kerkering as they do. But it wasn’t always a quality he had. When Kerkering first started working with his private pitching coach, Oscar Espada, he was a 15-year-old introvert with good stuff, but not much conviction behind it.

Espada, who is now the baseball coach at St. Petersburg College, set out to change that. He decided to keep it simple for his pupil. He gave him three rules.

“Rule No. 1 was, ‘Don’t be a [expletive],” Kerkering said. “Rule No. 2 was, ‘Don’t suck.’ Rule No. 3 was, ‘Don’t break rules one and two.’”

Added Espada: “I told Orion, you can’t be up there and be a [expletive] because you don’t have your fastball. Or your slider isn’t breaking. You have to go up there and pound the strike zone, attack hitters, and do it with what you have that day.

“Whether you’re throwing 95 or 75, you can still go out there and get guys out. If you don’t have your A-stuff, it doesn’t mean, ‘Oh, I’m done.’ It means, ‘OK, now I’ve got to battle.’”

Kerkering took it to heart. He wrote “Rule number one” under the brim of his cap throughout high school and college. Every time he found himself in a jam, he’d take a step off the mound, take off his cap, look at the brim, and get back to work. It helped him stay centered.

It still helps him stay centered. Kerkering said that when he’s on the mound, he still lives by Espada’s three rules. It’s a mentality that fits well with the Phillies’ pitching ethos. When they drafted him in 2022, amateur scouting director Brian Barber saw a “warrior who was not afraid of anything.”

“That’s the type of mentality that you want to see in the bullpen,” Barber said earlier this month. “If you brought him in with bases loaded, with one out, he wasn’t going to shrink. He was a type of guy who knew what he had, knew how to execute the pitches that he has, and just was not afraid.”

After establishing those three rules, Kerkering started to throw with more confidence. There was conviction behind every pitch. When he first started working with Espada in 2017 at his facility in Sarasota, Fla., they’d work alone at 6 p.m., every Wednesday.

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But over time, that changed. Espada’s 5 p.m. clients started staying late to watch Kerkering’s lessons. His 7 p.m. clients started arriving early to do the same thing. It became a spectacle. They wanted to see Kerkering’s notorious weapon: his slider.

Espada suggested that Kerkering add the slider because he had trouble throwing a curveball. He told him to throw it just the way he threw his fastball and draw the shape he wanted his slider to have with his long fingertips.

“It’s that little twist of the wrist that’s almost like — getting that thumb over his index finger, instead of rolling it out the hand,” Espada said. “So there was no need for him to try to spin his fastball a certain way. It was just going to come out of his hand.

“When we looked at video, I’d tell him, ‘Your fastball is a fastball on the outside corner until it becomes a slider. Because until about 20 feet away, you would still think it was a fastball away.’ And it would almost just disappear on hitters.”

Espada said it took less than a year for Kerkering to get the pitch down. As the pitcher grew taller and stronger, the slider only got sharper. Espada had to find local high school catchers to sit behind the plate, because he wasn’t able to catch it anymore.

Once Kerkering had that pitch, it was matter of throwing it with conviction — which is where the three rules came in.

“Because he throws it with so much conviction, from the same arm slot, as a hitter, you really don’t know what to expect,” Espada said. “That slider would have a ton of knees buckling.”

It still does. The Phillies hope to see more weird swings and buckling knees off that slider in the postseason. And Espada hopes to be there to watch it all.

“He helped me get here,” Kerkering said of Espada. “He’s had a ton of impact on my life.”