NEW YORK — Kodai Senga’s season debut was going about as well as the New York Mets could have hoped Friday night: The right-hander had nine strikeouts through five innings against the Atlanta Braves, and the Mets, a win away from taking the top NL wild-card spot, held a six-run lead in what would be an eventual 8-4 victory.

Then Austin Riley hit a measly infield popup and Senga tumbled to the ground, writhing in pain, before the ball landed in first baseman Pete Alonso’s glove.

Senga ended up on his back, grabbing his left calf. A trainer rushed out to check on him. Senga stood up and limped off the field to an ovation from an appreciative — and concerned — home crowd.

Minutes later, the Mets announced Senga exited with a left calf strain.

For five innings, there was nothing to be concerned about. The shoulder injury that had placed the Mets’ ace on the injured list to start the season was a distant memory. The mechanical hiccups that the cautious Senga cited as the reason for his longer-than-expected recovery period were nonexistent.

His only rough patch surfaced in the second inning when he gave up a leadoff single to Travis d’Arnaud. Two batters later, Adam Duvall swatted a two-run home run to give Atlanta a 2-0 lead.

The Mets, keeping to their recent form, responded with three home runs in a seven-run third inning, clobbering Charlie Morton out of the game before he could record nine outs.

Senga, last season’s NL Rookie of the Year runner-up, didn’t give up another hit across his 5⅓ innings. He issued one walk to the nine strikeouts. He threw 73 pitches — six shy of his total from his fourth and final rehab start last Friday.

He was cruising. Citi Field was alive. The Mets were vibing again. That changed with Riley’s popup. In a few seconds, the Mets went from celebrating Senga’s return, a massive boost to the rotation for their playoff push, to wondering when he’ll pitch again. The answer could have a significant impact on their postseason hopes.