The Mets have nothing to lose but time.

It could be all too easy to flip the calendar to November, when this season is officially over and “better luck next year” is more comforting of a thought. But the Mets have a long, grueling month and a half ahead, secure in the knowledge that they’re playing for little more than personal pride.

So really, it doesn’t matter all that much that Chris Flexen got the loss Friday night, or that Amed Rosario is still finding his way around a major-league batter’s box. If the Mets are going to make this time count — and they’re trying — it means throwing these kids straight into the fire and never once looking back.

In Friday’s rain-delayed, 3-1 loss to the Marlins, this meant that Flexen, who began the year in Single-A, had the dubious honor of facing down the hottest hitter in baseball in Giancarlo Stanton. Pitch around Stanton? Oh, then how about Marcell Ozuna and his 91 RBIs? And since we’re talking outfield, let’s not forget that adding Christian Yellich to the mix gives the Marlins the most productive outfield in all of baseball.

And Flexen did it. He didn’t always look great. He spent as much time avoiding Stanton as pitching to him, and did give up another home run, to J.T. Realmuto, but to his credit, he looks to be learning. Better yet, he’s nowhere close to satisfied. He raged in anger in the dugout after giving up the homer, and though he got applause when he left in the sixth inning, he shook his head in disgust as he headed into the dugout.

He allowed three runs on five hits in 5 1/3 innings, with four walks and a strikeout. He continued to struggle with command, but managed to work out of a few tight spots against that packed Marlins lineup. And, well, this is the Mets necessary reality now. Before the game, Terry Collins said his veterans know the September youth movement “is coming.”

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“They’re seeing it now,” he said. “Obviously, the situation with Rosario — he’s going to play the majority of the games at short . . . Same with Dom [Smith] at first base . . . That’s the time to do it.”

Smith, who had a rest day, struck out in his pinch hit appearance, while Rosario managed two singles, boosting his average to .246.

As for Flexen, his ascent is more due to necessity and desire — thanks to a rotation beset by injury.

Apparently nobody’s fool, he walked Stanton the first two times around the order, but got in trouble in other ways. After walking Ozuna to lead off the second, he hung a curveball to Realmuto that looked out off the bat. The two-run homer was his 12th this season. Stanton earned his second walk with one out in the third and this time, it was Yellich who did the damage. Flexen threw his slider — a pitch he’s only fully developed this season — and it didn’t quite bite like it ought. Yellich’s double to the leftfield corner set up Ozuna, who drove Stanton in with a sacrifice fly.

The Mets responded in the bottom of the inning, but not enough. Three singles led to a run, with Wilmer Flores driving in Juan Lagares with two outs.