NEW YORK — Michael King defied what was normal for a pitcher returning from a long layoff when he threw strikes effectively for five innings in his return from the injured list last week.

The rust was waiting for him Tuesday at Citi Field.

King getting hit hard and often on Tuesday was at least as troubling for the Padres as anything else about an 8-3 loss to the Mets.

It was arguably the greatest concern to come out of the night.

That is because the Padres are virtually certain to make the playoffs. But what they do when they get there could be largely up to King.

The 29-year-old right-hander would be the presumptive starter in the second game of the team’s first postseason series. But that would also presumably be on the condition he is able to right himself and show he is a semblance of the pitcher who has been their best starter over the past two seasons when he is healthy.

It might be a lot to expect of a pitcher who has spent more time on the injured list than active this season. But given his excellence and the less-than-stellar seasons by Dylan Cease and Yu Darvish, expecting King to perform in late September and into October is imperative for the Padres.

“Michael King, we’ve got to pitch him,” Padres manager Mike Shildt said. “… It would be unfair if we didn’t pitch Michael, relative to success and health and what it means to our team. Wasn’t his best outing. I’m excited to see him in five days.”

King threw an abundance of pitches up and over the middle of the plate while lasting one batter into the fourth inning on Tuesday, and that batter hit the Mets’ fourth home run of the night.

“Terrible fastball command,” King said after allowing eight runs on 10 hits, both more than he had ever surrendered in a game. “All around terrible. Bad pitch selections, bad locations. Got to be a lot better.”

The Mets were up 5-0 after the first inning and 7-1 after the second.

Jackson Merrill homered in the first inning, and Jake Cronenworth did so in the second to get the Padres to 7-2.

Cedric Mullins led off the bottom of the fourth with a home run.

Eight of the 17 balls the Mets put in play against King had an exit velocity of at least 99 mph, and six were off the bat at 104 mph or higher.

King’s start a week earlier against the Reds, his first in a month and second in nearly four months, was not without an occasional lapse in command. He surrendered a pair of home runs and had another brought back by a spectacular Fernando Tatis Jr. catch above the wall. But he allowed just one other hit and walked one batter while throwing strikes at a 67% rate.

Tuesday’s start and the two others he has time to make before the end of the regular season were considered part of a ramping-up process after being out so long.

King had a 2.59 ERA, sixth best in the NL, when he was sidelined after 10 starts with a nerve impingement near his right shoulder in mid-May. He came back to start Aug. 9 against the Red Sox but was bothered by knee pain, departed after allowing two runs in two innings and was placed on the injured list before his next scheduled start.

After throwing 63 pitches against the Reds, the hope was he would take that up another dozen or so Tuesday.

He ended up throwing 57 pitches before Kyle Hart took over.

Hart retired the next six batters. Yuki Matsui began the sixth and worked 1⅔ scoreless innings before Bradgley Rodríguez got through the eighth without allowing a run.

The Padres did not do much offensively after the early homers. Ramón Laureano’s two-out double in the fourth inning was their only other hit against Mets starter Clay Holmes. Sean Manaea relieved Holmes to start the fifth inning and allowed a single to Manny Machado in the sixth and a homer by Freddy Fermín in the eighth before finishing off the game.

The loss knocked one game off the Padres’ lead over the Mets, who sit in the sixth and final NL playoff spot, four games behind the Padres.

The two teams directly behind the Mets — the Diamondbacks at 1½ back and the Giants at two — had just begun playing each other Tuesday when the Mets completed their victory.

A cushion of at least 5½ games with 11 to play makes the Padres a virtual lock for the postseason. FanGraphs.com had their chances at 99.7% at the start of Tuesday night.

As for their chances of playing deep in October, what King does in his next start, scheduled for Sunday in Chicago against the White Sox, might provide a hint.

“I’ve got a lot to prove in my next one,” King said. “So it’ll be a grind for the next five days to make sure it doesn’t happen again.”