BOSTON — An already rough series against their rivals got even worse for the Yankees on Saturday, when DJ LeMahieu sat with a tight groin.

LeMahieu, who has exceeded all expectations since signing with the Yankees in the offseason and become their most consistent hitter this year, felt something in Friday night’s loss to the Red Sox, manager Aaron Boone said before Saturday’s game at Fenway Park.

“He felt something during the game [Friday] night and was able to finish the game,” Boone said. “He felt OK, [but] we want to make sure and see where we are at.’’

LeMahieu underwent an MRI exam, and the results will help determine whether he will end up heading to the injured list.

“I don’t think it is certainly major, but it’s enough that we want to get testing done,’’ Boone said.

Boone and Brett Gardner had similar feelings about the outfielder’s sore left knee before Gardner finally went on the IL after missing five straight games.

If LeMahieu does end up being shelved, he would have plenty of company there.

In addition to Gardner, the Yankees’ offense is also missing Gary Sanchez (groin) and Giancarlo Stanton (knee). And the Yankees have also been without Aaron Judge (oblique), Aaron Hicks (back), Miguel Andujar (shoulder) and Didi Gregorius (elbow) for significant chunks of time this season.

And that doesn’t include the oft-injured Jacoby Ellsbury and Greg Bird.

Boone said LeMahieu was unavailable for Saturday’s game, regardless of the MRI results.

The Yankees are off on Monday, so if LeMahieu sits again Sunday, he would have three straight days of rest before the Yankees open a homestand against Arizona on Tuesday.

They have weathered just about all the other injuries, but much of that has been because of LeMahieu, who didn’t seem to have a regular spot in the lineup when the Yankees signed him to a two-year, $24 million deal.

Instead, LeMahieu was expected to split his time at first, second and third base, playing five times a week so other players wouldn’t be forced to play too much.

But LeMahieu quickly has become invaluable, both offensively and defensively.

His approach and production at the plate has impacted the entire lineup and his ability to play above average defense at not only second base — where he was a Gold Glove winner with Colorado — but also at first and third has allowed Boone to use him on an everyday basis, whether it’s to give other players a day off or fill in for extended periods when others have missed stretches of time due to injury.

LeMahieu’s numbers at the plate are on par with his breakout season with the Rockies in 2016, when he led the National League in hitting.

He’s already set a career-high in RBIs (71) and matched it in homers (15), which he had last season in Colorado.

In June, LeMahieu was named Player of the Month and his OPS of .898 trailed only Judge’s .954 among Yankee regulars.

On Saturday, the Yankees filled the void left by LeMahieu’s absence with Luke Voit at first, Gleyber Torres at second, Gio Urshela at third and had Edwin Encarnacion as the DH.

But what’s helped make the Yankees’ lineup so successful is being well-rounded, which has no doubt been aided by the fact the players haven’t been overused and it has remained deep.

That will be tested again for as long as LeMahieu is out.