Jordan Montgomery’s knee was “pretty stiff” on Monday according to Aaron Boone after the lefty was drilled by a 102 mph comebacker from Xander Bogaerts in Sunday’s loss to the Red Sox.

“He’s doing all right,” Boone said before the Yankees lost to the Blue Jays, 3-0, at the Stadium. “We’ll see how he moves around. We feel like, structurally, everything is good.”

Montgomery crumpled to the ground and was in obvious discomfort following the play before he was able to get up and stay in the game after several warm-up pitches.

Given the lack of length the Yankees — and much of the rest of the league — are getting out of their starters, losing Montgomery with just one out in the top of the first would have added stress to an already taxed pen, but he managed to pitch into the fourth inning before Clarke Schmidt took over.

After the game, Montgomery said he expected to be able to make his next start, which would be scheduled for Friday in Baltimore. How he recovers and what he’s able to do before Friday will determine whether that proves to be realistic.

He’ll spend the next couple of days trying to get the stiffness out of the area.

Marwin Gonzalez was in the lineup for the first time on Monday night, getting the start at shortstop, with Isiah Kiner-Falefa and Josh Donaldson off, Gleyber Torres at second base and DJ LeMahieu at third.

Asked why Gonzalez was at short instead of Torres, who is expected to still play some games at the position, Boone said he preferred to keep Torres at second.

“I just want to keep Gleyber at second for now,’’ Boone said. “He’s gonna have to play short at some point, but I liked what I saw from Marwin, physically, this spring. That’s one of the things we were most excited about [with Gonzalez]. He moved really well and his range was good.”

Gonzalez made his first play on a Lourdes Gurriel Jr. pop-up down the left-field line to lead off the top of the first, where Joey Gallo nearly ran into him.

After Donaldson pinch hit for Gonzalez in the seventh, Torres moved to short.

Kyle Higashioka slammed seven homers during a torrid spring training but, so far, hasn’t been able to have the same production in the regular season.

He went 0-for-4 on Monday and is 1-for-11 to open the year.

“He just hasn’t clicked yet,’’ Boone said.

Ron Marinaccio’s good start to his career continued. The right-hander replaced Michael King with two on and two out in the eighth and struck out Matt Chapman. He ended up tossing 1 ¹/₃ shutout innings.

Nestor Cortes will take the mound Tuesday in his first step in trying to prove he can replicate what he did a year ago, when the left-hander emerged as a surprisingly consistent starter.

Asked what he believes helped Cortes make the leap from journeyman to someone the Yankees could rely on, Boone said Cortes was “better conditioned.”

“As a result, he sharpened up his stuff,” Boone said. “He’s done a great job, with his experience, of learning how to be a better pitcher.”

Giancarlo Stanton has gotten off to a promising start, with a three-hit game on Sunday and RBIs in all three of the Yankees’ games against the Red Sox.

“I think he’s just become so mentally tough and focused,’’ Boone said. “I feel like it’s been an evolutionary process in New York for him. He’s learned how to get himself prepared to block out any noise and I don’t think that stuff affects him. A star player like that, with an MVP, to come and play here, you’ve got to have pretty broad shoulders and he does.’’