PITTSBURGH — Frankie Montas reverted to the form Sunday that caused concern during his minor league rehab assignment. 

In his second appearance since returning from the injured list, the seasoned right-handed pitcher allowed two home runs in the opening inning and was pulled after completing the fourth inning during the Mets’ 12-1 defeat to the Pirates at PNC Park.

Overall, Montas allowed six earned runs on seven hits with five strikeouts and one walk.

Montas pitched to a 12.05 ERA in six minor league starts while rehabbing a high-grade lat strain. 

But Montas shined in his season debut for the Mets, with five scoreless innings against the Braves last Tuesday.

This time he faced eight batters in the first inning. 

“It was mainly the first inning where I missed my locations a bit,” explained Montas, who found himself trailing 5-0 before retiring the third batter. “Beyond that, I believe I made some solid pitches.”

After the bullpen was called upon for seven innings due to Paul Blackburn’s lackluster outing and a 1 ½-hour rain delay the previous day, the Mets needed Montas to provide more innings.

A night earlier, David Peterson failed to complete five innings. 

“I wanted to go out there and go as long as I can,” said Montas, who threw 86 pitches in his four innings. “I feel like [Richard] Lovelady did a real good job picking me up, but I have got to pitch in another five days and I have got to continue working on things.” 

Montas’ next turn will come during the Subway Series, most likely on Saturday.

Montas spent an injury-plagued two seasons with the Yankees. 

On this day Oneil Cruz’s two-run homer in the first inning placed Montas in a 4-0 hole after Ke’Bryan Hayes had delivered a two-RBI single.

Tommy Pham followed Cruz’s blast with a homer. 

“[Montas] couldn’t put hitters away,” manager Carlos Mendoza said. “He gets two outs and he gets ahead of Hayes and he’s not able to finish that at-bat. He puts the ball in play and then the pitch [a cutter] to Cruz. I think it’s more like pitch selection there. That’s not a good pitch, a pitch that he hammers. Same thing with [Pham], he’s a pretty good four-seam fastball hitter and once he gets ahead he leaves a pitch up in the zone for a homer.”