The bullpen has largely been a strength of the Mets this season, but it was a weak spot Wednesday night.

Allowing at least seven runs to the Cardinals for the second straight game — as well as the 18th time overall this season — the Mets relievers struggled to contain St. Louis in a 9-2 loss that was exacerbated by another meek team performance at the plate.

In front of a sparse crowd at Citi Field, the four Mets pitchers who took the mound combined to give up 11 hits, four walks and three home runs in the Mets’ third loss in their past four games.

“Obviously, the last two nights, not the way you want it,” Mets manager Carlos Mendoza said following the loss. “It starts on the mound with starting pitching. We haven’t done that the past couple of nights. And then you get in a hole and then offensively, it’s going to be a challenge. We need starters. We need the offense. We need to play defense. We need to play consistent games.”

It took the Cardinals until the third inning to break through in the series opener Tuesday, but the visitors wasted no time Wednesday.

As a result, the Mets, who continue to ride these periodic bullpen games, trailed 4-0 by the third inning for the second game in a row.

Austin Warren required an early mound visit after giving up a run four batters into the game.

Finishing with two earned runs, two walks and one strikeout on 33 pitches through the first inning, Warren had the Mets chasing the game from the get-go before another night of cold bats ensured the team was never able to recover.

While David Peterson replaced Warren and pitched for the majority of the game (3 ²/₃ innings), the 6-foot-6 hurler was not at his sharpest either. With two outs and two strikes on Masyn Winn in the third inning, Peterson ultimately fumbled the leading count with three straight balls and walked him.

On the very next pitch, Nelson Velázquez blasted a 92.4 mph sinker right over the plate for a two-run homer.

David Peterson looks down at the ground after giving up a two-run homer to Nelson Velázquez who round the bases in the third inning of the Mets’ 9-2 loss to the Cardinals at Citi Field. Robert Sabo for New York Post

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“It’s hard to describe, [Peterson is] a good pitcher and we’ve seen flashes, especially coming out of a bullpen,” Mendoza said. “I think it’s just outings where whether it’s a walk, whether it’s pitch selection, not executing, and today was one of those. I look at him as a guy that we’re going to need to get big outs for us, and I’m confident in Peterson even though it’s been hard for him. Continue to trust him and continue to work with him.”

The Mets managed to load the bases in the bottom of the third, largely thanks to Brett Baty’s leadoff single and stolen base. Cardinals pitcher Andre Pallante then hit Luis Torrens and walked Juan Soto.

Jared Young, however, grounded out to end the inning and strand all three.

Nelson Velázquez (38) celebrates with teammate Masyn Winn after hitting a two-run homer off David Peterson in the third inning of the Mets’ loss to the Cardinals. Robert Sabo for NY Post

While Peterson wasn’t trusted enough to make it to the end of the fifth, finishing with seven hits, six earned runs, two walks, two homers and one wild pitch, the Mets got a couple of scoreless innings out of Cionel Pérez and Jonathan Pintaro before the latter gave up a home run to Alec Burleson in the ninth inning.

Despite being requested by the media, Peterson left the clubhouse before reporters entered the locker room.

When Francisco Alvarez was asked if he’s sensed any frustrations from Peterson over what has been a rough start to the season, the Mets catcher denied the notion.

“No, no, Petey is a strong-minded person,” Alvarez said through a translator. “There have been points in his career where he’s had tough stretches, but he always finds a way to get out of there. I know as long as he continues to work, he’ll get out of it and by the end of the season, he’ll have the numbers that he wants to have.”