Police are investigating an allegation of common assault against Jose Mourinho following 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.
A spokesman for Manchester United confirmed they are aware of the allegation but made no further comment.
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."