LA's James appeared to strike Pistons centre Stewart in the face as they battled for a rebound, leaving him with a large cut near his right eye, BBC Sport reported.
Stewart then repeatedly had to be held back by team-mates and coaching staff as he tried to retaliate.
Both were then ejected from the game after an official review.
"Everyone in the league knows LeBron's not a dirty guy," said James' Lakers team-mate Anthony Davis. "In fact, when he knew he hit him, as soon as he did it, he looked back at him like, 'Oh, my bad. I didn't try to do it'."
The ejection – which took place in the third quarter – was only the second of James' 19-year NBA career.
"It was a tough play with Isaiah," said Pistons coach Dwane Casey. "His eye got cracked all the way open and he was upset for a reason. I don't think James is a dirty player, but again, it got them going."
Davis top-scored for the Lakers with 30 points and Russell Westbrook added 26 as the Lakers came back from 17 points down in the second half.
Elsewhere on Sunday night, the Los Angeles Clippers beat the Dallas Mavericks 97-91, while the Phoenix Suns recorded their 12th consecutive win with a 126-97 victory over the Denver Nuggets.
The Chicago Bulls picked up their fourth win in five games by narrowly beating the New York Knicks 109-103, and Andrew Wiggins inspired in a 32-point performance for the Golden State Warriors in their 119-104 win over the Toronto Raptors.