The former Manchester United team-mates got into it during the dying moments of the first half of the tie which was won 2-1 by Inter.
Lukaku and Ibrahimovic were involved in a verbal joust before they bumped heads having not seen eye-to-eye, which resulted in both being booked by referee Paolo Valeri.
As both were booked and pulled apart, Ibrahimovic can be heard yelling at Lukaku "Go do your voodoo s***, you little donkey".
The tension was palpable upon the half-time whistle with a livid Lukaku trying to chase Ibrahimovic down the tunnel - appearing to insinuate that the 39-year-old had made a comment about his mother.
The 27-year-old could be heard telling the AC Milan striker "f*** you and your wife, you little b****" and "you want to speak about my mother!?" as his Inter team-mates had to hold him back.
After the interval, tempers had calmed down but Ibrahimovic found himself in trouble just 13 minutes after the restart when he was sent off for a second bookable offence.
The veteran Swede was shown his second yellow card of the match for a trip on Inter defender Aleksandar Kolarov - in an incident that changed the whole game.