Grealish now has four goals this season and expressed his satisfaction at finally getting his landmark first goal in the competition.
"It's about time isn't it?!" he said humorously. "I'm over the moon to score. I've got to go in there now and thank Phil for the assist.
"I am over the moon to score but it's not about me. Obviously, I want to score and I want to assist, but the main thing to do tonight was go through to the next round and we did that.
"It was a difficult game. We knew it was going to be that. We finally broke them in the second half and our quality shone through."
The 26-year-old also spoke about the importance of patience to break down Peterborough, and looked ahead to the remainder of the FA Cup as well as this Sunday's Premier League clash with Manchester United.
Watch the entire interview via the video at the top of the page.