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Ibrahimovic, 39, fired AC Milan into the lead with a superb finish in the 31st minute at the San Siro.
Ibrahimovic and Lukaku squared up when the players left the field at half-time, with the 27-year-old Belgian striker pulled away from the confrontation.
Both players were booked during the incident, but tensions rose with what Ibrahimovic and Lukaku appeared to say to one another.
Twitter user Italian Football TV claimed to have watched the footage '100 times' to find out exactly what was said in the war of words.
And they suggest that Ibrahimovic called Lukaku a "donkey," with his former teammate wasting no time in firing back by calling him a "b***h."
Lukaku, who left United in 2019 to sign for Inter, fired back an expletive response directed to Ibrahimovic and the Milan star's wife.
When the Belgium international walked into the tunnel, Italian Football TV reports that Lukaku allegedly said: "You want to speak about my mother, you son of a b***h? I'll shoot you in your head."
Milan were reduced to 10 men in the second half as Ibrahimovic was booked again in the 58th minute for a foul on Aleksandar Kolarov.
Former Tottenham star Christian Eriksen came off the bench late into the second half and scored a fantastic free-kick to seal a 2-1 win for Inter in injury time.
Inter will now progress to the semi-finals of the Coppa Italia and will wait to see who their opponent is once the remaining quarter-final matches take place.
Featured Image Credit: BT Sport/Twitter
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