The fifth-year combo guard was among a number of people overheard screaming at each other inside the Celtics' locker room following the Heat's victory in Game 2 of the Eastern Conference finals on Thursday, according to multiple reports.
Reporters on site in the Orlando bubble also noted it sounded as if objects were being thrown inside the locker room.
The loss dropped the third-seeded Celtics to 0-2 in the series against the underdog Heat, the lowest remaining seed in the playoffs at No. 5.
Smart left the facility without speaking to reporters, while fourth-year Celtics forward Jaylen Brown addressed his teammate's demeanor.
"He plays with passion, he's full of fire, and that's what I love about him most, to be honest," Brown said postgame, according to Ben Golliver of The Washington Post. "He has that desire and that will, and we need him to continue to have that.
"There's ups and down in families all the time, but we embrace each other for who we are. Who Marcus is, I love him for it. So we've got to get ready to come back, take that same fire, and add it to Game 3."
Kemba Walker dismissed the notion that anything had happened at all.
"Nothing happened in the locker room," Walker told reporters, according to Yahoo Sports' Keith Smith. "I don't even know what you guys are talking about. We'll be fine. We'll be fine."
Celtics head coach Brad Stevens also downplayed the situation without going into specifics.
"Guys were emotional after a hard game, hard loss," Stevens said of the reported incident.
Game 3 is set for Saturday at 8:30 p.m. ET. Stevens believes his team will have to find harmony within the borders of the court if it's to have a shot at a series comeback.
"We're not beating this team if we're not completely connected on both ends of the court," he said. "So we've got to get back to being that. ... We're going to have to fight to get back into this series."