The centre-back saw a loss of possession capitalised on by the Rossoneri for the sole goal of the fixture in injury time.
Ghana international Nicholas Opoku left Dacia Arena with his head in hands as Udinese and Milan sold out a Serie A thriller on Sunday.
For what was otherwise a solid display in just his second start and fourth game of European football, the centre-back, who joined Udinese from Tunisian outfit Club Africain in the summer, saw Milan capitalise on his loss of possession to score a dramatic winner in the seventh minute of injury time.
With the Zebrette having pushed men forward and looking to build an attack in the quest for a possible last-gasp winner, the Ghanaian gave away the ball with an over-hit control around the centre line.
Milan mounted a counter in return and it was Alessio Romagnoli who emerged hero of the night with the crucial goal.
Unable to get back onto his feet after seemingly picking up an injury in the challenge that saw him lose possession, Opoku had to be helped off the pitch, bringing a disappointing end to what would have been a good day at the office.
The result has moved Milan to fourth on the league table, with Udinese just one place above the relegation zone on the 17th position.
Ghana coach James Kwesi Appiah will be keeping a close eye on Opoku’s injury situation.
A long layoff for the former Berekum Chelsea man will be another big blow to the Black Stars boss, who last week saw Leicester City defender Daniel Amartey suffer an ankle injury which will keep him out of action for four months.