United said on their website that it looked like a thigh problem but Netherlands coach Guus Hiddink suggested it was a knee injury, with some media reports speculating he could be out for six weeks.
"I'm not a doctor, but it doesn't look good. It looks like he twisted his medial ligament, but it could be a rupture as well," Hiddink told reporters
Such news would be a big blow to compatriot Louis van Gaal, whose stuttering United side have been riddled by injuries this season as they attempt to recover from last term's dreadful showing.
Blind had made a decent start to his Old Trafford career as a defensive midfielder, also being employed by Van Gaal as an emergency centre back.
United, currently seventh in the Premier League, will again have to reshuffle their pack as they prepare to visit Arsenal on Saturday.
(Editing by Ian Chadband and Mark Meadows)
