DENVER — In baseball, hierarchy permeates everything, from seating on the team charter to choice of a uniform number. And even though Amed Rosario had long been anointed a boy king in the Mets organization, he was not above the rules.

So back in spring training, when Rosario requested his choice of uniform number, he was greeted with a compromise, No. 61. It was something less prominent than his request, but more appropriate for a player in his first big-league camp.

Should he reach the big leagues, Rosario was promised, he’d be given his first choice. Which is why Tuesday night, in one of the most anticipated debuts by a Mets prospect in recent memory, Rosario wore No. 1.

“I really am at a loss for words with this,” Rosario said through a translator, before an eventful debut that ended with his botched backhand on a tough play that led to a 5-4 walk-off loss to the Rockies.

With the game tied in the bottom of the ninth, and Charlie Blackmon aboard after a leadoff walk, DJ LeMahieu sent a hard-hit ball to the left of second base. Had Rosario maintained his position, it might have been a double play.

But before the ball was hit, Rosario and second baseman Neil Walker changed the coverage of second base. If Blackmon broke, it would be Rosario’s bag to cover.

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When Blackmon took off, Rosario held his ground as long as he could before covering second, the proper play in that scenario. But LeMahieu’s grounder was perfectly placed.

“When you’re caught, you’re caught,” manager Terry Collins said.

The next batter, Nolan Arenado, singled to center to win it. Afterward, a few of Rosario’s new veteran teammates went to console the rookie, assuring him it was a fluky play.

“It was actually pretty difficult,” Rosario said of the play, which capped off an otherwise successful debut.

Rosario went 1-for-4, with his first big-league hit coming when he flashed his speed by legging out an eighth-inning grounder to short. On the play, he advanced to second on a throwing error. At the end of the inning, Yoenis Cespedes and Asdrubal Cabrera went over to hug the prospect.

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Lefty Steven Matz lasted just five innings, departing down 3-2 after allowing a go-ahead three-run shot to Arenado. The Mets rallied to take the lead on Jay Bruce’s solo homer in the eighth, only to be doomed by a pair of bloop hits against Jerry Blevins that allowed the Rockies to tie it, 4-4.

But for the Mets, the focus has long gone past wins and losses, and shifted toward shaping what comes next. For the Mets, that means watching Rosario’s progress.

“He’s the future of the organization,” shortstop Jose Reyes said. “He’ll be the shortstop for a lot of years here. For me, it’s not a problem at all. I’m happy to help him out with whatever he needs.”

The 21-year-old Rosario will cut into the playing time of Reyes, who once walked this same path. Reyes’ debut in 2003 at age 20 carried similar fanfare and proved to be the beginning of a career that includes four all-star selections.

Reyes is no longer that player, though he has served as a mentor for Rosario since last season, when the two crossed paths at Double-A Binghamton. Since then, the two spoke throughout the offseason, in spring training, and this season as Rosario hit .328 at Triple-A Las Vegas to earn his promotion.

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Before the game, Reyes took Rosario on a form of freshman orientation, showing him everything from the clubhouse cafeteria to the computer used to request tickets for friends and family.

“It’s been huge for me just being able to have that sort of special human being,” Rosario said. “He’s really been there a lot for me since last year, when we were able to start developing that. It means a lot to me.”

Later, Rosario would lean on that kind of support again. Before leaving the field, Bruce counseled “don’t beat yourself up.” Walker insisted “that’s a hard play for anybody.” Collins declared “he’ll be fine.”

“It really means a lot that they’re there for me like that,” Rosario said, his new No. 1 hanging near him. “But that’s just part of the game. You shake it off. I’ve already forgotten it. I’ll come in tomorrow with a new mind-set. And a lot of faith.”