Grealish is being linked with a move to Manchester United.
"On Grealish's future, that's difficult to say," O'Neill told Stats Perform News. "He's a big fans' favourite and Villa is his football club as well.
“Does he have the ability to play for Manchester United? Certainly, the natural talent to play for any of the big clubs – there's no question about that."
He added: "He's very strong; he's much stronger than he was five years ago, a quality player, hard to rob."