But an Inter spokesman has since told Sky Sport Italia that the claims are baseless, and denied any wrongdoing on the part of Mourinho.
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GMP have contacted United to request CCTV footage of the area and have confirmed a complaint has been made. "Shortly before midnight last night a man reported that he had been punched in the face outside Old Trafford. We are investigating an allegation of common assault."
United would only confirm they are aware of the allegation and are making no further comment.
Mourinho was jeered throughout the match by United fans, who sang "You're not special anymore".
Mourinho famously announced his arrival on the English stage with an ostentatious sprint down the Old Trafford touchline in 2004 after his unfancied Porto side eliminated United from the Champions League. Since then, his relationship with Sir Alex Ferguson has developed into mutual respect, and last night Mourinho admitted the best team won.
"I'm not angry," said Mourinho when questioned by Italian journalists. "You're angry and frustrated when your team doesn't play, when it doesn't express its qualities, or when it's afraid of playing. This is the kind of match after which, if anyone has anything to criticise my team about, they would have to kill me first because Inter were fantastic tonight, but it's not easy to play at Old Trafford and unfortunately luck wasn't on our side."