Jobless Paul Mitchell, 27, dashed on the pitch and whacked the star from behind in the match at Birmingham City because he thought he was a "n*b".
The Birmingham supporter was today snapped staring straight at the camera as he made his way to the city's magistrates court.
JPs heard how Mitchell has been forced to move with his family out of Birmingham following "death threats" over the incident.
He today pleaded guilty to assault and invading the pitch ten minutes into the match on Sunday.
His lawyer said: "This is a high profile case, captured on national television.
“My client’s young family have had to leave their home and move out of Birmingham in fear of serious harm or death.”
'IT COULD HAVE BEEN WORSE'
The court was told Grealish at first thought he had been punched by another player as the drama unfolded during Sunday's derby.
A statement by the footballer read out by the prosecutor said: "Jack Grealish was shocked and scared because if this person was brazen enough to get onto the pitch, what else could he do?
”He didn’t know person who assaulted him and didn’t antagonise Birmingham City supporters.
“It could have been worse had the person had a weapon.”
Mitchell, who has been a season ticket holder for over 20 years, hadn't been drinking at the game with his dad and hit Grealish "as a joke", the court heard.
Jonathan Purser, prosecuting, said: "Jack Grealish felt a push in the back and a hook to his lower jaw. He had just won a corner and had back to stand containing Birmingham City supporters.
“The TV footage had captured the defendant running onto the football pitch, Mr Grealish with back to him and had name on back of his shirt.
“Footage shows him taking a running swing, jumping on the back of Grealish and launching that punch.”
The court was told the attack on Grealish was "totally out of character" for Mitchell, who has been banned from attending Birmingham matches.
His solicitor said: "He cannot explain what came over him yesterday morning. His initial foolish intention was to just go onto the pitch and whip up the crowd."
He said Mitchell, who used to manage a pub, has a two-year-old daughter with his partner with another child on the way.
Mitchell, of Birmingham, will be sentenced later this afternoon.
Last night, we revealed he had received death threats from outraged footie fans.
It is claimed the married dad-of-one saluted fellow Birmingham fans as he was led away.
Grealish was poleaxed in the 10th minute of yesterday’s Championship game, shown live at lunchtime on Sky Sports.
The midfielder, 23, later scored the only goal of the game and appeared to be shoved by a Birmingham steward in the celebrations.
Shocked ex-Villa ace and pundit Lee Hendrie said: “That is literally the worst thing I’ve seen on a football pitch.”
Security is set to be reviewed amid a day of shame for football, with Birmingham facing a points deduction and fan ban.
Pundit Gary Neville said any more incidents could see the return of fences at grounds.
Birmingham City later apologised to its local rivals and promised to review security at its St Andrew’s stadium.
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