The teenage winger has suffered an ankle injury, with the initial diagnosis ruling him out for the remainder of the campaign.
Advertisement
Advertisement
Mantato, 18, suffered the setback in U21 action against Chelsea this month. He is now sporting a protective boot as he begins his rehab.
The teen made his senior debut against Wolves before Christmas.
England U18 international Mantato is now aiming to get himself fit in time for United's preseason tour squad, which will include more youngsters than normal with the World Cup taking place.