Ibrahimovic appeared to catch the Real Salt Lake defender with his arm which prompted tensions early on and a booking for the Swede.
He went on to score the winner in a 2-1 victory for Galaxy, and made a point of squaring up to Onuoha in a very provocative celebration.
Zlatan's attempt to apologise in the Salt Lake dressing room after the match then got rejected with tensions still running high.
"He came in to apologise after the game, because from 60 minutes in, he's saying he's going to do me, he's going to hurt me for that game," Onuoha said after the match.
"This is a guy who is the face of MLS, as he calls himself, and he plays that way on the field.
If someone comes in and says that to me, I'm not going to accept his apology. It's unacceptable.
Ibrahimovic's explanation was less than apologetic:
"What happens on the field stays on the field. I like to feel alive. 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 activate me.
"They need to activate me or else it becomes too easy. I know me. When I get angry I feel good."
Clearly Onuoha did not "feel good" in the same way as a result of Zlatan's anger.