ST. LOUIS — Blade Tidwell was a young star of Mets spring training, boasting a blazing fastball, a bigger frame and — one day in March during a Grapefruit League game — an immaculate inning: three strikeouts on nine pitches.

And now he will get a chance to do it for real.

The Mets will promote Tidwell to the majors Sunday, when he’ll start against the Cardinals, manager Carlos Mendoza announced Friday.

“He earned it,” Mendoza said. “We know the stuff is there. We saw flashes of it in spring training . . . When we needed a spot start, he put himself in a position and he’s getting the call.”

Tidwell, 23, has been among the Mets’ most interesting pitching prospects since they drafted him in the second round in 2022. Last year, he dominated with Double-A Binghamton, advanced to Triple-A Syracuse, struggled and seemed to figure it out late.

After impressing in his first major-league camp, he returned to Syracuse. Some of his overall numbers — including a 5.00 ERA — are ugly, but the Mets are encouraged by his peripheral statistics, including 37 strikeouts (to only 10 walks) in 27 innings.

His fielding-independent pitching (3.17) and batting average on balls in play (an unusually high .369) suggest he has been a bit unlucky in how many runs have scored against him.

“Even though some of the numbers probably are inflated a little bit, the underlying [numbers are good]. He’s throwing strikes, he’s using all of his pitches,” Mendoza said. “He put on some really good weight [over the offseason]. You gotta give him credit. He worked extremely hard, put himself in a position where he came in and pretty much impressed everyone in camp because of how he was looking physically and how the ball was coming out.”

Tidwell, who reached 98.5 mph with his fastball in his most recent outing, is listed at 6-4 and 207 pounds but almost certainly is larger than that now.

“He was telling me he added 20 pounds of muscle this offseason and I was like, dude, you look like [NFL tight end] George Kittle right now. You’re massive,” Christian Scott said during spring training. “He is the best I’ve ever seen at manipulating the baseball. Whether he’s throwing a cutter, a gyro slider, a sweeper, the way he’s able to manipulate it and get numbers on his pitches is the best I’ve ever seen. Using his talent to the best of his ability. And also, when he comes in and he gains 20 pounds of muscle, you can see how his work ethic is.”

This is the third time in three weeks that the Mets have inserted an extra starter during a stretch of many games without a day off, a concerted effort to give all of their rotation members — especially Kodai Senga — extra rest.

Justin Hagenman and Brandon Waddell each made an appearance, both behind an opener. Now Tidwell will get a try. With every Thursday off the rest of the month, they should be good for a while after this.

Yet to receive an opportunity is the other big prospect with Syracuse, righthander Brandon Sproat, the organization’s top-ranked minor-leaguer.

Sproat has a 5.48 ERA in six outings, a tough start after a rough introduction to Triple-A last year. He has mixed a few decent-to-good starts with a few clunkers.

“That’s exactly right: a little inconsistent,” Mendoza said. “That’s part of the development: finding a way to get through starts where maybe it’s the second time through the lineup, third time through the lineup. I don’t think he’s gotten that far, but just making adjustments. It’s not just trying to throw as hard as you can. The hitters will adjust to you. That’s why he’s there in the minor leagues, continuing to develop.”

Sproat has struck out 17 (and walked 12) in 23 innings, an unusually low rate for someone whose fastball touches triple digits. Even in pitching six shutout innings in a game last month, he fanned only one batter.

As he dominated a couple of other minor-league levels last season, conversely, he had 110 punchouts in 87 2⁄3 innings.

“We’re not surprised. We’re not surprised,” Mendoza said of the strikeout frequency. “But again, that’s part of how you’re going to use your pitches. How are you going to get ahead? How are you going to put [hitters] away? When there’s traffic, are you looking to get a ground ball or strikeouts? That’s part of the learning. We got really good people down there who are helping him.”

Pete Alonso won NL player of the month for March/April, MLB announced. Luisangel Acuna won NL rookie of the month.

“Proud of both of them,” Mendoza said. “We know Pete is a big part of this team and how good we played out of the gate. And Acuna, taking advantage of his opportunity, creating traffic, getting on base — and when he gets on base, how electric he is and how much pressure he puts on other teams, continue to play really good defense. It was good to see those guys get rewarded.”

The Mets’ bullpen shuffle continued, with Austin Warren called up from Triple-A Syracuse and Ty Adcock optioned back after one game .  .  . Neither A.J. Minter (left lat) nor Danny Young (left elbow) has decided whether to have season-ending surgery but could make a decision any day now, Mendoza said.