MOOSIC, Pa. — The biggest fright of Justus Sheffield’s season displayed exactly why the Yankees cherish him.

The top pitching prospect in the system and the Yankees’ No. 3 prospect overall, according to MLB.com, felt tightness in his left, pitching shoulder in the fourth inning Friday, his second start since getting called up to Triple-A Scranton/Wilkes-Barre. What had been Sheffield’s showcase, with senior director of player development Kevin Reese on the scene, became a scare in an instant.

Manager Bobby Mitchell walked to the mound to pull his starter, who had tried in vain all day to get his shoulder loose, with each pitch only tightening it. Still, Sheffield, his arm dangling stiffly, didn’t want to go anywhere.

“I expect big things from him,” Mitchell told The Post this weekend. “He’s very, very competitive, which is going to help him. He didn’t want to leave [Friday] night, but we had to take him out. He’s just driven.”

Sheffield didn’t make a scene but made his frustration known.

“I didn’t want to come out at all. I knew going into that game, my goal was going out there and pitching seven, pitching a complete game,” said the 22-year-old, whom the Yankees acquired in the 2016 Andrew Miller trade. “I love the game, I love to compete. The worst games are the four games between my starts.”

There will be more than four games this time. Sheffield was placed on the seven-day DL and has seen a doctor and taken strength tests he says went well. He insisted he will be back soon and miss just one start. The Yankees have called the DL trip “precautionary.”

Still, it’s a pitcher’s shoulder, which was wrapped in ice Saturday.

“Just the word ‘shoulder’ is a concern,” Reese said. “I’m hoping that he’s right. … Right now [the concern level is] as low as it can be given the body part.”

A week earlier, the Yankees promoted Sheffield, who had mowed down Double-A Trenton competition to the tune of a 2.25 ERA with 39 strikeouts in 28 innings. The call to the big leagues’ doorstep was long-awaited; Sheffield pitched all last year with Trenton. The southpaw from Tennessee, with a mid-90s fastball, a nasty, hard slider and improving changeup was ready for the next step. Still, the good news that didn’t come last season proved drawn-out this year.

Sheffield and the Thunder got in late from a road trip to Portland, Maine, not arriving in New Jersey until about 6 a.m. on May 3. A confused Sheffield awoke to hundreds of Twitter messages — the report had leaked — and no confirmation.

He didn’t have to be at the ballpark until 6 p.m. He tried to ignore it all, and when he finally arrived at the park, he got changed and went through his warm-ups.

“I had no word or anything, but everyone on Twitter was like, ‘Congratulations’ about my promotion and things like that. Finally [manager] Jay Bell called me in. He was like, ‘You probably already know, but … ’”

Sheffield said he considers himself a “bulldog,” a person who demands the ball and wants to finish games. He’s listed, generously, at 6-foot and 200 pounds, a build that recalls Johan Santana before anyone on the Yankees’ current staff. His mentality follows, constantly trying to prove himself.

“I’m not big in stature — like all the media people say, ‘Not big in stature’ — but it’s all good. I love that,” Sheffield said. “That adds logs in my fire.”

It’s that feature that helped convince the Yankees to promote him.

“His stuff is at a quality where he doesn’t have to nibble around hitters,” Reese said. “You don’t even need to strike out 10 guys. He’s showing that.”

Bumping him up a rung left him one call from the majors, where former teammates are expediting the franchise’s future. He cited Domingo German, Gleyber Torres and Miguel Andujar as contributors he played alongside last year, similarly top-ranked prospects who got the call and ran with it. He admitted seeing friends on the big stage makes him want it even more. With a Yankees staff that has already lost Jordan Montgomery and contains a seesawing Sonny Gray, it makes sense that Sheffield would insist the shoulder injury isn’t serious.

“It kind of puts things in perspective — what can happen in a year,” Sheffield said. “Gives you the drive to keep working harder. You never know when you’re going to be called.”

There’s another, more urgent reason Sheffield is eager to pitch again. Through a few spring trainings, he’s become close with CC Sabathia, who still texts him bits of advice. The 37-year-old Sabathia told The Post’s George A. King III last week that if the Yankees win the World Series, he’s likely finished.

“That’s my guy. Honestly, I’d like to be his teammate before he retired,” Sheffield said. “That would be cool for me just because, ever since I came over here, he took a big-brother role, helped me out on and off the field.

“… I’d really want to be his teammate before he leaves. But if he does leave, that’s a good thing because we won the World Series.”