A drunken thug chanted: "Fergie, Fergie, shut your mouth" after punching Sir Alex Ferguson between the legs, a court heard yesterday.
Celtic fan Kevin Reynolds, 44, is thought to have had a go at the Manchester United boss because he once played for rivals Rangers.
Yesterday he admitted assaulting Sir Alex and headbutting and shouting racial abuse at a community police support officer.
Reynolds, who has 105 previous convictions and had only been out of jail five days, was warned he was heading back behind bars.
Homeless Reynolds, described by magistrate Daphne Wickham as a "fighting drunk", had downed several strong lagers and half a bottle of vodka before his attack at Euston station in London, City of Westminster magistrates' court heard.
Sir Alex, 65, who had arrived from Manchester for a charity event, on Monday thought he was begging.
Then Reynolds, from Fife, suddenly punched him in the groin.
Carol Tod, prosecuting, said: "Mr Ferguson asked, 'what the hell are you doing?' He replied, 'I'm sorry Fergie. I didn't know it was you'." He then stuck out his arms, chanting."
Police support officer Peace Toluwa arrived and Reynolds shouted racist abuse and headbutted him.
Adeola Oluwu, defending Reynolds, said he was joking around and hit Sir Alex while shadow boxing.
Ms Wickham said: "I do not think Sir Alex Ferguson saw this as a joke. I think it has to be prison."
Afterwards witness Bob Walkey, 45, told the Mirror how Sir Alex doubled up in pain but did not retaliate.
He said: "I was amazed by his calm. Anyone else would surely have retaliated, but he showed astonishing restraint."
He added: "The thug hit him and began mocking him. Although Sir Alex was in pain and doubled up, he remained calm.
"I'd seen this loser staggering around and he had an air of menace. I saw a policeman nearby and sent him over."