The pair clashed twice, before Ibrahimovic scored the 78th-minute winner and celebrated in Onuoha's face. Onuoha, 32, says he rejected the Swede's apology in the dressing room.
"Over here it is just a show for him. A lot of people are scared of him - but he is just a mere mortal," he said.
"First, he was very arrogant. Secondly, I thought: 'What is this guy playing at? This is not the way you are supposed to do it, especially someone who is meant to be the face of the game here and the game globally. I wasn't a fan before - and I'm definitely not a fan now."
The first flashpoint came in the 60th minute, when 37-year-old Ibrahimovic was booked for dragging Onuoha to the ground by his neck in an off-the-ball incident.
Onuoha said the former Manchester United striker then made the threats to him following another challenge shortly afterwards.
"This time he hit me on top of head. I was again thinking: 'What is going on here?'" Onuoha told BBC Sport.