England midfielder Grealish is on the verge of a £100m move to Manchester City and Villa have identified Cantwell as a potential replacement.
Yet Norwich do not want to sell the former England under-21 international having already lost Emi Buendia to Villa in a £33m transfer which could be worth £38m.
But if Villa beat that previous deal with a bigger offer - £40m - Norwich will not be able to resist the cash.
If Grealish, 25, smashes the British record for a transfer fee, the club know they will have to pay over the odds for a replacement.
Cantwell, 23, who came through Norwich's youth system, wanted to leave Carrow Road at the start of last season as the club was in the Championship.
But now Cantwell is no longer in a rush to leave the club as they have returned to the Premier League.
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