Now he has returned, United's medical staff will ascertain the extent of the damage and how long he will be sidelined.
United are now second in the table, behind neighbours Manchester City on goal difference, and seven points ahead of Liverpool.
Romelu Lukaku has been declared "fit" by Belgium, but Manchester United will be sweating on his possible involvement in World Cup qualifying action.
Statistics show just how important he is to Jose Mourinho's team and reveal why he will be badly missed during his recovery from injury.
A visit to Swansea City in the fourth round of the Carabao Cup follows and the month is rounded off with a home match against Tottenham and the home return with Benfica.
"He is still a very young player but he has coped easily with the expectations and whatever Man United put on him".
With Fellaini set to sit out United's trip to Anfield, Mourinho is likely to turn to the club's reigning Player of the Year, Ander Herrera, to help provide cover in the middle of the park.
Manchester United have a huge game at Liverpool this weekend and Mourinho does not want any risks taking on a nothing game, but is not getting involved with the decision.