Normally, a performance like the one Jordan Montgomery delivered in Tuesday’s 6-3 loss to the Mets at Citi Field would merely be a blip on the radar, since the left-hander has been so solid for much of the season. 

But Montgomery’s inability to get out of the third inning — while tying a season-high five runs, in his worst start of the year — was part of a somewhat troubling trend among the Yankees’ rotation.

The pitching staff has not been as good over the past month as it was during most of its lights-out first half — and that includes the starters.

“We’re not gonna be perfect all year,’’ Montgomery said. “If you have 30 starts, you’ll have five stinkers. I hope this is my one.”

Though Gerrit Cole has been good and Nestor Cortes is rolling along after briefly faltering earlier in the month, the Yankees have seen Jameson Taillon pitch poorly, Luis Severino land on the IL with a strained lat and his replacement, Domingo German, have a rough outing in his season debut in Houston last week. 

Rarely has Montgomery been hit harder than he was at Citi Field on Tuesday.

Staked to a 2-0 lead in the top of the first thanks to back-to-back homers by Aaron Judge and Anthony Rizzo, Montgomery went out in the bottom of the inning and got torched.

He was pounded for four extra-base hits in the four-run bottom of the first. Starling Marte took him deep with one out, Francisco Lindor doubled then scored when Pete Alonso followed with a double of his own.

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And Eduardo Escobar finished the damage in the 33-pitch inning with a two-out, two-run homer.

Montgomery briefly recovered with a three-up, three-down second, but he didn’t make it out of the third.

Marte got him for a leadoff double. Lindor then hit a routine grounder to third, but Josh Donaldson made a poor throw to first that hit Lindor in the helmet and bounced into right field, allowing Marte to score and Lindor to get to second.

Montgomery walked Alonso and struck out Mark Canha before being removed with Escobar coming to the plate again.

“I wanted to be out there, but I sucked,” Montgomery said of his 71-pitch performance. “Obviously, I needed to be pulled.”

It was just the fourth time in 20 starts Montgomery allowed more than three runs and took his ERA to 3.50, the highest it’s been since his first start of the year, when Montgomery gave up three runs in 3 ¹/₃ innings against the Red Sox on April 10.

“I felt like I was making decent pitches, but they must’ve had a really good game plan against me, and it’s a good lineup,’’ Montgomery said. “I’ve just got to be better. I need to be more down in the zone. If I wanted a swing-and miss, they just kept fouling it off.’’

The rough night against the Mets left Montgomery with some ugly numbers in the Subway Series. In four starts, he’s allowed 15 runs over 15 innings (9.00 ERA), and the Yankees have lost all of his outings against their crosstown rivals.