Police are investigating an allegation of common assault made against Jose Mourinho after a Champions League tie.
It has been alleged the Inter Milan manager punched a supporter as he left Old Trafford stadium after his team's defeat by Manchester United.
Police have contacted United to request CCTV footage of the area and have confirmed a complaint has been made.
Inter Milan have denied the allegations, saying Mourinho had a "very calm post-game experience".
A statement from the club read: "At the end of the game, the Portuguese tactician gave interviews to the European media, signed autographs, watched the penalties of the Roma v 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 dressing rooms and hence it was a very calm post-game experience."
The fan claims he was assaulted outside the ground
A spokesman for Manchester United confirmed they were aware of the allegation but made no further comment.
The fan is alleged to have chanted, "You're going home, you're going home" to the former Chelsea manager.
The man later went into Stretford police station, close to Old Trafford, to report the alleged attack.
A Greater Manchester Police spokesman said: "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 allegation came after Mourinho watched his team crash out of the Champions League after Nemanja Vidic and Cristiano Ronaldo scored for United at the start of each half.
Mourinho has come under attack from the Italian media for his failure to get Inter through to the last eight teams in the competition.
The plane taking the Inter party back to Milan left Manchester at 0025 GMT.
The fan, who is not from the Greater Manchester area, did not report any injuries to police after the alleged attack and did not need treatment.
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