The Swedish star conceded that he was left injured from the fight with the American defender and stated he is a regular target of racial chants in Italy.
The pair made headlines in November of 2010 when the United States international started a fight with Ibrahimovic in a practice session at Milanello after he suffered a nasty challenge from the Sweden international.
It is reported that Onyewu, currently on the books of Sporting Lisbon, and Ibrahimovic exchanged blows before being separated by teammates.
At the time, Milan vice president Adriano Galliani played down the incident, which he described as a 'lively fight', but Ibrahimovic, who is a black belt in taekwondo, admitted it was not a normal brawl between players.
"Onyewu and I almost killed each other," said Ibrahimovic in his autobiography, titled 'I Am Zlatan Ibrahiovic', according to the Swedish press.
The former Inter star claimed that he suffered a broken rib as a result of the fight. The 30-year-old also said that racial chants are normal in Serie A matches and that he is used to being called a "gipsey" by fans.