First-half goals from Sergio Aguero and Rodri were enough for the Citizens despite Mbwana Samatta's header, but Grealish believes that the corner awarded for City's second goal should have been given as a goal kick instead.
Speaking to Sky Sports News after the game, the 24-year-old said: "Let's be honest, the winner is a goal that should never have been a goal because it wasn't a corner.
"He just says the guy was already off the pitch when it hit him, but he clearly wasn't. We came in here at half time to try and get an explanation but he didn't want to give one.
"It's obviously gutting, because a club like Aston Villa haven't won anything for quite a long time now. We come here today for a final and we lose by a goal that should never have stood."
VAR was in use at Wembley for the final, however was not called upon to review Rodri's header.