The Inter Milan boss is at the centre of a police investigation after a man - believed to be from East Anglia - filed a complaint of common assault.
Bad mood: Jose Mourinho walks off the field after his side's defeat at Old Trafford on Wednesday night.
Mourinho is alleged to have struck the fan outside Old Trafford after his side lost 2-0 to Manchester United in the Champions League.
Steven Mace, who was waiting to get players' autographs shortly before midnight on Wednesday, told The Sun: 'Mourinho was giving a TV interview and us lot were giving him a bit of argy-bargy singing, 'Go home Mourinho'.
'He finished his interview and just came barging over towards us with this look on his face. We thought he was going to give autographs, but he walked straight towards this one fella.
'He chanted in a sarcastic voice, 'Go home Mourinho'? And the next thing he hit this guy. It wasn't a clenched hand more like a backhand across the face.
'This other guy was so shocked he didn't react. Then Mourinho calmly walked off. It was a moment of madness from him. I couldn't believe it.'
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Another United fan Dean Vickers also told The Sun: 'I was standing next to the guy who got hit.
'We were singing 'Go home Mourinho', and he didn't look impressed. He walked up and sneered, 'Go home Mourinho'? then 'Bang', this fella got a backhander.
'He was in his 40s and in no way a hooligan. The guy went pale and was holding his face. He was in shock'
Greater Manchester Police last night revealed that the former Chelsea manager is being investigated.
Inter have denied the allegations. A spokesman said: 'As far as we are concerned, nothing happened.
'The coach was surrounded by several hundred people and Jose was simply making his way back to it so they could go to the airport.
'We are obviously aware of this allegation as it is being reported in the Italian media, but as far as we are concerned nothing happened. There is no truth in it.'