Grealish was king-hit by the supporter - who came from the home end at St Andrew's - less than 10 minutes after kick-off.
The 23-year-old was immediately surrounded by his concerned teammates but was able to pick himself up and continue as the fan was led away by police, blowing kisses to the crowd as he was escorted off the pitch and arrested.
And Grealish later silenced St Andrew's when he collected Neil Taylor's pass to shoot across Lee Camp and find the bottom corner from 16 yards.
Former Villa midfielder Lee Hendrie, on duty as a pundit during half-time, called for a severe punishment and suggested the game would have had to be abandoned had Grealish suffered a serious injury.
Hendrie said: "I'm all for the rivalry, I'm all for the banter, but that is literally the worst thing I've seen on a football pitch.
"Jack Grealish, one of the key players - what if he breaks his jaw and has to go off the pitch? That's one key player who is out of your side. If that would have happened, I think the game would have had to have been abandoned.
"A fan running on to the pitch and taking someone like Jack out, that could have been anything, there's knife crime... anything could have happened out there."