DJ LeMahieu showed a side of himself that rarely comes out as his prolonged slump continued Thursday night.

Usually calm and collected, the infielder fired his helmet down the tunnel that connects the Yankees’ dugout and their clubhouse. Clearly disgusted with a third-inning strikeout that saw him drop to his back knee, LeMahieu then chucked his lumber into the bat rack.

“We’re playing for a lot, man,” Aaron Boone said of LeMahieu’s uncommon outburst after a 10-2 loss to the Mariners. “These guys have a lot of pride and have done it for a long time at a high level. And when you go through some struggles for an extended period, guys are angry. That’s part of it. Am I worried? No, he’s as tough as they come mentally, physically. We got to just continue to work alongside him and try and unlock him.”

LeMahieu, thrust back into the leadoff spot after two days on the bench, went 0-for-4 on Thursday with two strikeouts. He is now hitting .228/.286/.382 with seven home runs, 23 RBI and an 84 wRC+.

He’s also striking out 27% of the time, which is roughly double his previous high as a Yankee.

“It’s beyond frustrating,” LeMahieu admitted Friday when asked about his helmet toss, “but that’s not gonna stop me from working and competing and trying to be the best I can for this team.”

Boone, Brian Cashman and hitting coach Dillon Lawson have attributed LeMahieu’s struggles to a problem with his load. The veteran agreed that that’s part of the problem, but, contrary to his nickname, he’s never been prone to thinking too much about his mechanics.

“That’s one of those things that I’ve really never thought of,” said LeMahieu, who played again Friday against the Rangers. “I just go up and hit. I don’t really think about that stuff. I’m not a mechanical guy or mechanical hitter.”

Still, LeMahieu and Yankees coaches have been doing deep dives on his cut, and he’s been trying to make adjustments with cage work and on-field hitting sessions before games.

“Moving in the right direction, hopefully,” LeMahieu said. “It’s been a grind. So just trying to find solutions.

“I haven’t really hit this bad in a while. I don’t know if ever. And it’s coming at a bad time for our team.”

Indeed, the Yankees’ scuffling offense would welcome the return of LeMahieu’s batting champion form or at least a fraction of it.

For what it’s worth, Boone still believes there’s a dangerous hitter in there.

“One thing that’s always come pretty easy to him is the ability to wake up and hang out a line drive,” the manager said. “So when that’s not happening at as consistent a level as he’s accustomed to, you gotta start tinkering with some things or exploring things and figuring things out. Again, I think he has a good run in him. We just got to get him there and unlocked.”

“I don’t think, mechanically, he’s all that far off. It’s very subtle.”

DONALDSON’S DAY OFF

Josh Donaldson has also posted poor numbers, tallying a .127/.200/.413 slash line, six homers, eight RBI and a 62 wRC+ after going 0-for-3 on Thursday. Afterward, Boone was asked if the third baseman could use a few days off, similar to what LeMahieu received.

Boone didn’t dismiss the idea and ultimately held Donaldson out of Friday’s lineup.

“Played a bunch in a row and just piecing the puzzle together,” Boone said when asked to explain the decision. “Just felt like today was a good day to have him down.”

On Thursday, Boone said that he felt Donaldson has been “getting his swing off,” but he wanted to see him be “more precise at the ball because I do feel like there’s some pitches that he’s in position to impact and kind of going through a stretch of just missing.”

Boone added that the Yankees had faced three straight right-handed starters who are especially tough on righties.

But Donaldson, a former MVP, has hardly hit since becoming a Yankee last season, and fans have lost patience with the 37-year-old. Many have wondered if the Yankees will designate Donaldson for assignment — a la Aaron Hicks — and he heard loud boos throughout the Seattle series at Yankee Stadium.

That included Thursday, when Donaldson, typically a strong defender, initiated a two-error play in the third inning.

“You feel for anyone going through that when that’s the case, because you know what guys put into it,” Boone said of the jeers. “You know how much they care. You know that’s not necessarily fun to go through. Everyone handles it a little bit different. But it goes with playing here sometimes. It goes with playing in the big leagues sometimes and the big stage and the pressure cooker. It’s part of it and most guys understand that. So I don’t necessarily love it, but it’s part of the deal.”

MINOR INJURY UPDATES

Boone didn’t have anything new on Aaron Judge (right big toe) on Friday, but Nestor Cortes (rotator cuff strain) is about a week away from getting on a mound.

Willie Calhoun’s quad strain is between a Grade 1 and 2.

()