Grealish later scored the victor to make it 1-0 to Villa on 66 minutes.
For his part, Grealish said he wasn't fully aware of the incident as it unfolded, saying: "I was just unaware at the time".
"There's rivalry and stuff in football, but I don't think there's any place for that in football". It was unbelievable, in front of our fans and after what happened in the first half. It was something I have dreamed of since I was a little kid. "Even with the punch, it ended up being one of the best days of my life".
The Professional Footballers' Association added that they've seen a rise in fan-related violence.
"We kick off on Sunday at 12 o'clock for a reason, to keep the people out of the pubs", Villa manager Dean Smith said.
You can tell he enjoyed it, diving into the crowd to celebrate the victor, but Jack really, really enjoyed the moment.
Mr Whistance, defending, said: "It has brought shame upon his family and the club he has supported as a child".
"It's a situation no player should ever be faced with", the EFL said.
Grealish and Aston Villa ended up having the last laugh though as he ended up scoring the game-winning goal in just the 67th minute.
In the League Championship game on Sunday between the city rivals, Paul Mitchell ran onto the field and landed a punch on the player's face.
"We will be working with the clubs, the leagues and the police to discuss what collectively needs to be done to protect players and officials on the pitch".
Birmingham said Mitchell would be banned for life from St Andrew's and Arsenal later pledged the same punishment would be imposed on the pitch invader at their match.
Birmingham said it would assist investigators and review stadium safety procedures.
"What happened has no place in football or society". It is the unfortunate reality for the club that, while there was no shortage of astonishment at the nature of the attack on Jack Grealish, there will be less surprise from the footballing world that a Birmingham fan was involved.
While Aston Villa and Grealish himself have not yet publicly commented on the incident, several other high-profile members of the footballing community have spoken up.
Passing sentence, magistrate Alison Fisher told Mitchell: "This type of behaviour is completely unacceptable - a view shared by your own football club".
A man has been arrested for allegedly punching a football player on the field of play during a match in England.
Despite the three points, the main talking point was the disgusting act of violence perpetrated by the pitch invader.