Grealish, Villa's captain on the day, was attacked less than 10 minutes into a heated affair at St Andrew's when a man entered the field of play and struck him around the neck and jaw from behind.
Players from both teams checked on the midfielder's welfare and he went on to score the decisive goal in Villa's 1-0 victory, beating Lee Camp from just inside the box in the 67th minute.
Grealish said the ugly incident, which led to an individual being arrested by West Midlands Police, took him by surprise, but did not overshadow a proud day.
"I was just unaware at the time," Grealish told Sky Sports when asked about the attack.
"I think we had a corner or a throw-in and I was walking into position when I felt a whack around the side of my face. Obviously there's rivalry in football but I don't think there's any place for that really.
"I just tried to get on with my job in the end and I think I did. It was unbelievable to get the winner in front of the fans after what happened.
"It's the best day of my life to come here as captain of Aston Villa and score the winner. It's what dreams are made of, especially being an Aston Villa fan."
West Midlands Police confirmed during the game that a man had been arrested and was being taken to a custody block following the attack, while Birmingham apologised to the player and their opponents.
"We deplore the behaviour of the individual who committed this act and rest assured he will be banned from St. Andrew’s for life," a statement read. "The club will also support any further punishment this individual may face in the eyes of the law."
The club continued: "What happened has no place in football or society. Jack is a Birmingham lad and regardless of club allegiance should not have been subjected to this – there are no excuses."
Grealish's winner lifted Villa to ninth in the Championship, a point above their Second City rivals.