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Speculation surrounding the 24-year-old continues to build heading towards the summer transfer window.
A move to Old Trafford has long been mooted, with Ole Gunnar Solskjaer still in the market for added creativity, but any number of Premier League heavyweights could join the scramble for his signature.
Grealish has proven himself at the highest level this season, with the England hopeful starring for a struggling side.
As a boyhood Villa fan, he is determined to help his current club preserve their top-flight status.
It is, however, looking increasingly likely that he will be plying his trade outside of the West Midlands in 2020-21.
Heskey, who was at Villa as Grealish emerged through the ranks, expects that to be the case, telling BBC Radio 5 Live when asked if the playmaker is ready for the elite: "Oh yeah.
"When I was at Villa he was actually a young 15-year-old kid who would come and train with us every now and again and he used to do the same then when he was 15 as what he's doing now. So you knew he's got something special.
"I think Man U were looking at him then but he's a Villa fan and wants to do well for Villa and definitely got what it takes to play at the top four clubs.
"To be honest, it's a matter of when, not if he's going to leave."
Villa are understandably reluctant to part with a prized asset but are aware that they have a fight on their hands.
They will give Grealish a shot at major silverware when taking in a Carabao Cup final clash with Manchester City on Sunday, but are braced for interest in their captain to be stepped up.
Villans boss Dean Smith has said: "He wants to play at the highest level and he wants to win trophies.
"We have to work hard on Sunday to win that trophy.
"But staying in this league is one of the biggest things to help us keep him. Jack's future is down to what happens over the next three months."