Grealish's terrific 85th-minute strike from 25 yards broke the deadlock against Neil Warnock's side at Villa Park on Tuesday night.
The 22-year-old's goal delighted Prince William in the directors' box, who was sitting alongside former Villa striker John Carew.
According to Aston Villa manager Steve Bruce, the Duke of Cambridge came into the dressing room at half-time and delivered a motivational team talk which did the trick.
Bruce said post-match: "We're honoured that the future king is an Aston Villa fan. He popped into the dressing room tonight, did the team talk, picked the team and it worked wonders.
"It was good to see him. We've given him a ticket for the next game!"
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