Paul Mitchell entered the field of play less than 10 minutes into the Second City derby between Birmingham City and Aston Villa before punching the side of Grealish's head.
The suspect's solicitor, Vaughn Whistance, said his client had no explanation for the incident.
"He cannot explain what came over him yesterday morning," Whistance said of Mitchell. "His initial foolish intention was to just go onto the pitch and whip up the crowd."
Prosecutor Jonathan Purser told the court that the Aston Villa captain - who managed to remain in Sunday's match and later scored the winning goal - felt lasting pain but suffered no substantial injury from the blow.
"I cannot help but feel how lucky I was in this incident," Grealish added in a statement read by Purser.
"It could have so much worse had the supporter had some sort of weapon."
Meanwhile, Birmingham City condemned the incident after the match, confirmed that Mitchell has been issued a lifetime ban from St. Andrew's, and said the club would support any further punishment "in the eyes of the law."
In the wake of both incidents, Professional Footballers' Association chief executive Gordon Taylor suggested clubs should be penalized with sanctions such as "fines, ground closures, playing games behind closed doors, and points deductions" to prevent supporters from committing similar crimes.
Additionally, Whistance said Mitchell's family has been forced to leave the "area" as a result of fear caused by online threats. The West Midlands Police confirmed it's investigating complaints regarding offensive social media posts.