Mets catcher Francisco Alvarez, Jose Iglesias and a trainer check...

Mets catcher Francisco Alvarez, Jose Iglesias and a trainer check on starting pitcher Kodai Senga during the sixth inning against Atlanta in an MLB game at Citi Field on Friday. Credit: Kathleen Malone-Van Dyke

It looked like a big addition. Now it feels as if it could be a major subtraction.

The streaking Mets appeared to grow much stronger on Friday night with the return of ace-in-waiting Kodai Senga. The righthander looked brilliant through five innings against Atlanta, but when he returned to the Citi Field mound for the sixth, things went very wrong.

Four pitches into the inning, he got a pop-up from Austin Riley, and as he cleared out of the way for first baseman Pete Alonso to make the play, Senga stumbled and went down between the mound and the first-base line. The crowd of 34,673 went silent and Senga reached for his left calf.

After spending more than a minute on the turf while being examined by team medical personnel, he walked gingerly to the dugout. The team announced the injury is a left calf strain and that he will have an MRI on Saturday.

Senga ended up the winning pitcher in an 8-4 victory as the Mets rode a grand slam by J.D. Martinez, a two-run homer by Mark Vientos and a solo shot by Francisco Alvarez — all in the third inning off Atlanta’s Charlie Morton — to their fifth straight win.

“It’s a tough piece to lose, but there’s no sense of dwelling on it,” Martinez said. “You know he’s going to do everything he can to come back, and hopefully he’s back by the playoffs.”

The Mets, who were 24-35 on June 2, have gone 31-13 since then. They have vaulted past Atlanta into second place in the NL East and hold the first National League wild-card spot.

But this is a brutal, emotional turn of events for the Mets. Senga was going to be a major lift, a starter who placed seventh in the 2023 NL Cy Young Award voting after winning 12 games and striking out 202. And that lift was coming without the expense of a trade as he returned from the shoulder injury that had sidelined him all season.

He looked like a sure difference-maker against Atlanta. Through five innings, he allowed two runs, two hits and a walk and struck out nine, recording a whiff to finish each of the five innings. His fastball touched 97 to 98 mph. He deployed his signature “ghost fork” effectively. He showcased a diverse collection of pitches that included a slider, cutter and sweeper and was impressively economical. His only blemish was a two-run home run by Atlanta’s Adam Duvall.

“He was pretty impressive,” manager Carlos Mendoza said. “Watching him pitch — the fastball, the command of his pitches, the splitter — he dominated today. You hate to see him go down like that, especially after what he’s been through this year. But people will step up if he’s down .  .  . We have to see what we’re dealing with.”

Alvarez said, “He was the same guy from last year — he was nasty.”

“[He] was great,” Martinez said. “The whole stadium, everyone, all the fans, everybody [saw it]. It’s a tough break, but .  .  . we’re going to keep staying positive and not look at the negative of this.”

But after this freak injury, no one can be certain when Senga will pitch again.

He already was in a position to get the win with the Mets ahead 8-2 going to the sixth. He had thrown only 69 pitches and was brought back out for one more frame, perhaps because the relievers had thrown 17 1⁄3 innings in the previous four games.

Senga’s return was to mean that the Mets would be shifting to a six-man rotation with an eye toward giving him five days of rest between starts. He was 12-7 with a 2.98 ERA in 29 starts in 2023, but his ERA was 4.61 in the three starts he made on four days of rest.

Tylor Megill was picked to become the sixth starter and will get the ball Saturday against Atlanta. When Senga wasn’t able to start the season because of injury, Megill was the original rotation replacement. The Mets have to hope that isn’t going to be the case again.

“It’s a big piece that we’re losing .  .  . He was a huge piece,” Martinez said. “The way he threw the ball today was amazing, and it was exciting.”

Notes & quotes: The Mets acquired reliever Ryne Stanek (6-3, 4.38 ERA, seven saves) from the Mariners for minor-league outfielder Rhylan Thomas .  .  . The Mets placed reliever Dedniel Nunez on the 15-day injured list with a right pronator muscle strain. Mendoza said, “It should be a short stint.” Reliever Eric Orze was recalled to take his spot in the bullpen .  .  . To clear the roster spot for Senga, righthander Adrian Houser was designated for assignment.

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