Francisco Alvarez’s walk-off homer gives Mets crucial win over Orioles

Alvarez had received the green light on 3-0 from manager Carlos Mendoza moments earlier.

Alvarez, who hadn’t homered since July 26, began the night with a dismal .447 OPS since the All-Star break.

“It’s been hard for him: 22 years old and struggling at the big league level,” Mendoza said. “He cares so much and he wants to win. At times when we are struggling offensively as a team he feels like he is responsible.”

The Mets won for the third time in four games and moved within 1 ½ lengths of Atlanta for the NL’s third wild card.

Monday started a 10-game stretch against strong playoff contenders — the Padres and Diamondbacks are also on the docket — that could have a large say in whether the Mets reach the postseason.

“We are fighting to make the playoffs, so I think the biggest thing is I helped the team a lot and I am happy for that,” Alvarez said.

This is one the Mets needed to finish after jumping to a 3-0 lead before the Orioles eventually tied it in the seventh.

David Peterson was on the verge of a gem before balking in a run and allowing a solo homer to Ramon Urias, both with two outs in the seventh.

Overall, the left-hander allowed three runs, two of which were earned, on six hits with eight strikeouts and one walk.

It was a fourth straight start in which Peterson — who leads Mets starting pitchers with a 3.00 ERA — has surrendered two earned runs or fewer.

“That was a good hitting team right there and he just went right after them,” Mendoza said.

J.D. Martinez’s two-run homer in the first gave the Mets a 2-0 lead. Mark Vientos singled before Martinez cleared the fence in right-center for only his second homer of the month.

The blast was his first at Citi Field since July 26.

With two outs in the inning, Starling Marte singled and stole second.

The stolen base was a positive sign from Marte in his second game back off the injured list after missing two months with a bone bruise in his right knee.

Tyrone Taylor (who started in left field a day after Brandon Nimmo departed with right shoulder discomfort) stroked an RBI single in the fourth that widened the Mets’ lead to 3-0.

Pete Alonso’s leadoff double started the rally before Alvarez drew a two-out walk and Taylor delivered.

Urias’ double helped the Orioles score an unearned run in the fifth.

Peterson’s pickoff attempt at second base sailed into center field, allowing Urias to reach third before Jackson Holliday’s ground out brought in the run.

Francisco Lindor and Alonso walked in the fifth, ending the outing for left-hander Trevor Rogers with two outs in the inning. Colin Selby entered to strike out Marte and keep the Mets’ lead at 3-1.

Peterson balked in the run in the seventh and Urias homered on the next pitch to tie it 3-3.

Ryan Mountcastle doubled leading off the inning and took third on a ground out before plate umpire John Tumpane, with two outs, signaled for the balk before the first pitch to Urias. Peterson said the pitch clock was about to expire and he was at fault for rushing his delivery — causing the balk — instead of stepping off the rubber.

The next pitch disappeared behind the fence in left-center.

Jose Butto and Edwin Diaz combined for two scoreless innings in relief.

Mendoza said he was discussing a plan for the 10th inning with pitching coach Jeremy Hefner as Alvarez was swinging on the game’s final pitch.

“I looked around and the ball was already in the air,” Mendoza said. “The feeling [was] incredible … guys jumping around and a good win for us.”