"He knows (why)," MacLean said after the Devils' 6-1 loss to the Sabres.
MacLean refused to say what Kovalchuk did to get benched, and he wouldn't say whether the star forward will be back in the lineup for Sunday night's game against the rival Rangers in New York.
"I'll make that decision in the morning," MacLean said.
Kovalchuk, who took part in the pregame skate on Saturday morning, was not in the Devils' dressing room after the game. Team president and general manager Lou Lamoriello didn't accompany MacLean to the postgame news conference.
MacLean refused to say whether his decision was related to hockey.
"I'm not discussing it," MacLean said. "It was my decision. I made it. He knows. I spoke to him and that's where it's going to stay."
While he hoped his decision to bench Kovalchuk was not a distraction to the team, MacLean couldn't explain why the Devils (2-5-1) played so poorly.
"I'm having trouble finding the answers for that myself," he said. "But ultimately it falls on my responsibility to get the guys prepared."
Zach Parise, who scored the Devils' goal on Saturday, found out that Kovalchuk wouldn't be playing just before the game. He had no comment on the decision.
"This is a step backward," Parise said of the Devils' effort two days after they shut out Montreal.
None of the players seemed to know why Kovalchuk wasn't in the lineup.
"You have to ask the guys who make the decisions," Devils captain Jamie Langenbrunner said.
Kovalchuk had two goals and three assists in the Devils' first seven games.
He signed a controversial 17-year, $102 million contract in the offseason that the NHL said circumvented its salary-cap rules. The Russian eventually signed a revamped 15-year, $100 million contract.
The NHL fined the Devils $3 million and took away two high draft picks over the next four years.