A January loan signing from Fiorentina, the midfielder has struggled for minutes during his time in England.
TMW says United paid 10m to take Amrabat on a six month deal, which includes an agreed permanent option. United can buy the Morocco international outright for 20m plus 5m in bonuses.
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However, United have decided to pass on the player's option.
Instead, Amrabat will return to Florence at the end of the season.