“Jack Grealish, they made an attempt to sign in January, Aston Villa knocked that back,” Castles said on the Transfer Window podcast.
“Obviously not wanting to lose their key player during the season when they’re trying to retain Premier League status.
“I think there is a real opportunity for Manchester United there in the summer.
“Grealish is not a player you will see staying indefinitely at Aston Villa if he continues this progress in his game. And you would expect Aston Villa to take a big transfer fee for him at some point.