Marouane Fellaini has undergone surgery in an attempt to resolve the knee ligament issues that have dogged him this season.
Jose Mourinho confirmed the news after Manchester United's 2-0 win over Huddersfield and expects the Belgium midfielder to be out of action for two months.
Fellaini has already had two lay-offs of six and four weeks since first sustaining a medial ligament injury in his left knee on international duty against Bosnia in October.
He limped off against Tottenham in midweek with a lateral ligament problem in the same joint just seven minutes after coming on as a substitute.