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. The fan was quickly surrounded by players and escorted off by stewards, blowing kisses to the crowd, as a dazed Grealish sat smiling on the St. Andrews field.
Grealish had the last laugh, firing home a 67th-minute victor for Aston Villa in a ferocious derby contest.
West Ham midfielder Declan Rice was quick to react to the incident, tweeting: "Absolutely disgraceful what just happened to Jack Grealish".
CLASH: Police hold back fans outside the ground (Pic: PA) SWOOP: Police use batons (Pic: PA) "What happened has no place in football or society".
"Whilst this incident falls within the remit of the Football Association, we will work with all the relevant parties to address the issue of player and match officials safety on the pitch and ensure the appropriate action is taken".
However, whatever the result ends up being we have already seen the talking point for this game and it has marred the whole derby.
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It's not our finest hour after the game the other day, but we've got a professional duty to fulfil. "We do it day-by-day, even more so in victory".
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"We deplore the behaviour of the individual who committed this act and rest assured he will be banned from St. Andrew's for life", read Birmingham's statement. "The club will also support any further punishment this individual may face in the eyes of the law".
Grealish stayed on the pitch to score the winning goal.
"Again, we apologise to Jack and all at Aston Villa Football Club".
Former Villa midfielder Lee Hendrie was working at the event as a pundit for Sky Sports, and said the incident went way beyond the boundaries of the long-standing rivalry between the two clubs. The worst thing I've ever seen on a football pitch.
"It's the best day of my life to come here as captain of Aston Villa and score the victor".
"I just tried to get on with my job".