A fan accused the former Chelsea boss of punching him after Manchester United's 2-0 win.
The Inter Milan statement read: "At the end of the game, the Portuguese tactician gave interviews to the European media, signed autographs, watched the penalties of the Roma vs. Arsenal game before saluting [Sir Alex] Ferguson in his office. After giving the last interview to Inter Channel, he entered the team bus that was parked outside the locker rooms and hence it was a very calm postgame experience."
The BBC quoted a Greater Manchester Police spokesman as saying: "Just before midnight on Wednesday 11 March 2009, a man reported he'd been punched in the face outside Old Trafford. Police are investigating this common assault allegation. Inquiries are ongoing."
The fan, who was not injured, reportedly chanted "You're going home, you're going home" at Mourinho.