ST. LOUIS — The Mets lost the game Sunday afternoon, but they didn’t necessarily lose the day.

They dropped their doubleheader opener against the Cardinals, 6-5, after falling just short in a bunch of run-scoring chances, finishing 3-for-15 with men in scoring position and leaving 12 on base.

Luisangel Acuna, who collected three hits and a walk, popped out to second base against Ryan Helsley to end the game, stranding the potential tying run at third base.

The Mets and Cardinals will play again at 6:15 p.m. ET as Tylor Megill faces Andre Pallante.

Down by four in the middle innings, the Mets had a chance to go ahead in the eighth, after Francisco Lindor’s two-run single made it a one-run game. Juan Soto walked to re-load the bases, but St. Louis lefthander JoJo Romero retired Pete Alonso (strikeout looking) and Brandon Nimmo (flyout) to escape the jam.

In addition to reaching base four times and upping his average to .313, Acuna played third base — for the first time at any level — and acquitted himself well in limited chances before Jesse Winker’s exit (right side discomfort) pushed him back to second base.

Acuna charged, fielded Nolan Arenado’s weak ground ball and fired to first on the run in his first chance.

“He was engaged with [infield coach Mike Sarbaugh] and the positioning,” manager Carlos Mendoza said. “I thought he was good.”

Acuna said through an interpreter: “I’ve been playing third base since spring training and I felt really good there . . . I’ve had this confidence since the team pretty much instilled it in me the first day that I got here. Ever since then, I’ve felt like I’ve been part of the team, so I feel really good.”

In his major-league debut, righthander Blade Tidwell got hit around for six runs in 3 2/3 innings.

He dealt with a ton of traffic on the bases — nine hits, three walks — and managed that mostly fine until the fourth inning, when St. Louis plated four runs on a pair of two-out, two-run singles from Brendan Donovan and Willson Contreras.

“We saw flashes of his potential, especially with the life on the fastball,” manager Carlos Mendoza said. “But we also saw that there is room for development, especially with the secondary pitches. At this level, you’re going to need pitches to put hitters away, but also you’re going to need secondary pitches to get back in counts.

“I thought the changeup wasn’t there today, with that many lefties there. He flashed a few sliders, some sweepers, but then he left a couple on the middle of the plate. Those are some of the things that he will continue to work on. But we saw flashes.”

Contreras also homered in the second inning, sending a slider on the outer third of the plate into the Cardinals’ bullpen in rightfield.

“That was a good piece of hitting,” Tidwell said. “I executed that pitch where I wanted it. Maybe it was the wrong pitch, maybe not. But he put a good swing on it.”

Tidwell lamented that he had trouble finishing batters off when he got ahead in the count.

“A lot of the weak contact was late in counts when I should’ve been able to put them away,” he said.

No matter the outcome, though, he achieved his dream of making it to the majors. Between games, he posed for pictures on the field with a couple dozen family members and friends.

“It was awesome,” he said. “Indescribable, really. It was everything I hoped for and more.”