PITTSBURGH — David Peterson took the mound Friday night as one of the last men standing in the quickly shortening list of healthy Mets starters.

It’s a heavy responsibility, with little room for error, and on this night, one (really) bad inning helped sink the Mets, who dropped the series opener to the lowly Pirates, 9-1, at PNC Park.

The Pirates scored four off Peterson in the second inning and never trailed. In an effort to shore up a badly taxed bullpen, newly recalled Blade Tidwell pitched the final 3 1⁄3 innings, allowing four runs and five hits.

The streaky Mets offense remained silent again, aside from Juan Soto’s fourth-inning homer. They went 0-for-9 with runners in scoring position and left eight on base.

“I felt like he was fighting,” manager Carlos Mendoza said of Peterson, who allowed five runs, seven hits and three walks with five strikeouts in 4 2⁄3 innings and has given up 10 earned runs in his last two starts. “There’s a lot of arm-side misses. The changeup is kind of just floating in the strike zone .  .  . It’s the same thing with the slider, [with a lack of] sharpness.”

The Mets (48-35) have dropped 11 of their last 14, partially keyed by an onslaught of injuries to a pitching staff seemingly held together by hopes, dreams and a carousel of minor-leaguers.

With the announcement Friday that Griffin Canning had season-ending Achilles surgery, the Mets have lost three starters to the injured list in the last three weeks. Sean Manaea also hit a snag when loose bodies were discovered in his left elbow, delaying his return. The bullpen, meanwhile, has had to navigate a series of abbreviated starts.

They could have done without that second inning, which came after Peterson needed only nine pitches in the first.

After the lefty walked Joey Bart with one out, Ke’Bryan Hayes and Jared Triolo followed with back-to-back singles, the latter of which drove in the game’s opening run. Alexander Canario then lined a ball to deep right that Soto got a late jump on, and the RBI double to the warning track put the Pirates up 2-0. Isiah Kiner-Falefa singled to left to drive in another run and Tommy Pham hit into a forceout at second to make it 4-0. Of the eight balls in play that inning, seven were hit at 101.7 mph or harder.

“I didn’t feel like I had my best command and wasn’t able to execute some pitches and they took advantage,” said Peterson, who’s seen his ERA jump from 2.49 in mid-June to 3.30. “I think it’s all stuff that we constantly check back in with. I didn’t necessarily stay on line very well and I got a little too rotational there . . . I think it’s trying to square that away and work on that again.”

The Mets got one back in the fourth when Soto smacked Mitch Keller’s knee-high sweeper 416 feet to center for his 20th homer and 11th this month.

The Pirates (33-50) added a run in the fifth on Hayes’ two-out RBI double off Peterson and three in the sixth on Bryan Reynolds’ three-run homer off Tidwell.