The Morocco midfielder has not travelled with the squad to Austria and is training alone back in Florence.
He is understood to have had his head turned by interest from other clubs and is not in the right frame of mind to play in the Conference League play-off first-leg fixture.
United have a long-standing interest in Amrabat and are weighing up a move, having not ruled out signing a new central midfielder regardless of outgoings before the deadline.
Liverpool have also been keen on Amrabat and, according to Sky in Italy, held talks with Fiorentina via intermediaries earlier this summer. They are also in the market for a player in his position before the deadline.
Amrabat is still very much part of Fiorentina’s plans for the season, but if a bid of around £17m to £21m (€20m to €25m) arrives then the club are expected to consider a sale.
Amrabat’s contract at Fiorentina expires next summer but the club have an option to extend it until 2025.