In an apparent attempt to join goal celebrations, another fan pushes an opposition player.
Grealish had the last laugh, firing home a 67th-minute victor for Aston Villa in a ferocious derby contest.
The incident occurred inside the first 10 minutes at St Andrew's, with a man making a beeline for the Villa captain and striking him on the head from behind.
"I was walking into position and then just felt a whack around the side of the face", Grealish said. Following the game, Grealish told ESPN, "Obviously there is rivalry in football, but I don't think there is any place for that".
Grealish was caught off guard - as were the other players in the area and security officials.
A 27-year-old man was arrested on suspicion of encroachment onto the pitch and assault, with hosts Birmingham announcing the individual will be banned for life.
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That's because Villa came away with the victory, clinching a 1-0 win on their rival's turf courtesy of a 67th minute Jack Grealish victor.
"It is the third time that a fan has managed to get on the pitch", he said.
Later on Sunday at a different match-up, a fan ran onto the field during a Manchester United and Arsenal game at Emirates Stadium. "The individual will also be banned from Arsenal matches home and away".
Sure, he probably just had a few choice words for Smalling, and no real harm done, but there's no way that fans should be able to get so close to the players with such ease.
"It is a criminal offence to enter the field of play and we will be working with the police, the relevant authorities and the club to ensure the appropriate action is taken". Football needs to stamp this out quickly. We were all watching and no one could believe what Paul did.
Stoke's former Scotland midfielder Charlie Adam tweeted: "Twice in three days we've seen players attacked by supporters, what is going on?"
Mr Whistance, defending, said: "It has brought shame upon his family and the club he has supported as a child".