Ibrahimovic, who scored the winning goal late on, received a yellow card after 60 minutes for grabbing the former Manchester City defender and throwing him to the floor, sparking a fiery confrontation.
After the match, the players continued the argument in the dressing room and Onouha later confirmed that he wasn't willing to make amends with the Swedish forward.
"He came in to apologize after the game, because from 60 minutes in, he's saying to me he's going to do me, he's going to hurt me for that game," Onuoha told reporters. "And this is the guy who's the face of MLS, as he calls himself, but this is the way he plays on the field.
"So I don't care. Someone comes in and tries to do that to me -- you don't say that on the field. I don't care. I'm not going to accept his apology. It's unacceptable."
Ibrahimovic had little regrets over the altercation and said that it helped him to perform better in the game.
"I like to feel alive," Ibrahimovic told reporters afterwards. "I like when it becomes duels and that, because sometimes -- not that I fall asleep, but I don't feel alive if they don't actually activate me. When I get angry, I feel good."