Brad Marchand had a really dumb reason for fighting Lars Eller

The first fight of Lars Eller’s NHL career left the Capitals center bloody and bruised. During Washington’s 7-0 win Wednesday over the Boston Bruins, Eller took a number of clean right hands to the face from Bruins winger Brad Marchand.

The scrap — which came with six minutes left in the third period — was seemingly random. Marchand was given a 10-minute game misconduct, leading to his ejection, and an instigator penalty.

So exactly what happened? Apparently, there are “unwritten rules” in hockey, too, not just baseball.

“His celebration was unnecessary,” Marchand said. “He took an angle in front of our bench and celebrated in a 7-0 game. So I just let him know.”

That’s right — Marchand was upset with Eller for … having scored a goal in a game. And then being a little too happy about it.

The Capitals were unaware of Marchand’s explanation but angry with the winger nonetheless, particularly on the heels of Tom Wilson’s 20-game suspension.

“I think it’s unfortunate for Tom that the league is making an example out of him,” forward T.J. Oshie said. “They set the standards, they want to get the dirty stuff out of the game. Which I think at least Tom’s play was on the ice and he was hitting the guy that had the puck milliseconds before.

“And then you see tonight the sucker punches that Lars took in,” Oshie said. “So they kind of set the standard. Marchand has a history, and we trust that they’ll do what they’re supposed to do and take care of business.”

Marchand has a string of controversial incidents in the NHL. Last season, the league intervened when Marchand kept licking opponents during the Bruins’ first-round series against the Tampa Bay Lightning, telling him he had to stop.

Bruins coach Bruce Cassidy stood up for his player after Wednesday’s game.

“That’s good,” he said. “He’s a proud guy. I think Eller celebrated a little on a 7-0 goal. I think that’s his prerogative and Marchy let him know that that’s not acceptable. That’s that and we move on.”

Capitals coach Todd Reirden called the fight “unjust,” while Oshie said he understood why Marchand wanted to fight Eller, but if the Capitals center didn’t want to engage, then he should have let it go.

“It’s a long season,” Oshie said. “We play them a lot of times. They could have handled that differently.”