Struggling David Peterson implodes again as Mets fall to Padres

David Peterson’s late-season doldrums are only deepening.

The Mets still have work that needs to be finished before they get too serious about a postseason rotation, but the lefty Peterson isn’t showing much these days to demonstrate he is worthy of such consideration.

Wednesday night he appeared ready to overcome a sloppy beginning, but then the fifth inning arrived and all was lost.

The culmination was Manny Machado’s grand slam that sunk Peterson and the Mets in their 7-4 loss to the Padres at Citi Field.

The Mets remained 1 ½ games ahead of the Diamondbacks for the NL’s third and final wild card.

The Reds and Giants are both two games behind the Mets.

Peterson, a workhorse for much of this season, represented the Mets in the All-Star game.

But since Aug. 6 (encompassing eight starts), he has been underwhelming.

Over that stretch, he’s pitched to a 7.58 ERA. In six of those starts he failed to complete six innings.

The series rubber game is Thursday, when the Mets will turn the rotation over to their youth movement: Jonah Tong is the first of three straight rookies scheduled to start for the team (with Brandon Sproat and Nolan McLean behind him).

Peterson allowed a single to Fernando Tatis Jr. leading off the game and walked Luis Arraez. Gavin Sheets’ sacrifice fly put the Mets in a 1-0 hole.

Pete Alonso’s two-out homer in the first got the Mets their first run.

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It was the third straight game with a homer for Alonso, whose blast was his 36th this season and gave him 119 RBIs.

Jake Cronenworth’s RBI single in the second gave the Padres a 2-1 lead.

Jackson Merrill singled leading off and advanced to second on Jose Iglesias’ groundout before Cronenworth delivered.

Starling Marte’s solo blast in the fourth tied it 2-2.

The homer was Marte’s ninth this season and moved him to 6-for-11 lifetime against Nick Pivetta. Marte hadn’t homered at Citi Field since May 30.

Peterson loaded the bases with one out in the fifth before Machado cleared the left field fence for his 14th career grand slam — the most among active players.

Machado launched a full-count curveball, only the second homer allowed by Peterson in his past seven starts.

The rally started with Cronenworth plunked leading off the fifth and a walk to Tatis. The ensuing batter, Arraez, bunted for a single to load the bases before Peterson went to a seventh pitch on Machado.

Juan Soto homered in the fifth to slice the Mets deficit to 6-3.

It was Soto’s 41st homer of the season, tying the career high he established last season with the Yankees.

Dom Hamel, in his major league debut, pitched a shutout sixth inning.

The right-hander surrendered three hits in the frame, but caught a break when Arraez was thrown out by Soto attempting to stretch a single into a double.

The tag was applied before the lead runner crossed the plate, negating the run.

Francisco Alvarez homered leading off the bottom of the seventh to pull the Mets within 6-4.

Soto missed a game-tying homer to left by inches later in the inning — his shot just missed hitting the foul pole — before striking out against Mason Miller.

Ramón Laureano homered against Ryne Stanek in the ninth to increase the Padres lead to 7-4.

Soto came to the plate in the ninth as the tying run and was retired by Robert Suarez to end the game.