Now he has returned, United's medical staff will ascertain the extent of the damage and how long he will be sidelined.
However, the hope is Lukaku will be fit to face Liverpool at Anfield on Saturday in a match that Mourinho will be under huge pressure to win on two fronts.
Manchester United striker Romelu Lukaku has been passed fit to face Cyprus in Belgium's final Qualifier for the 2018 World Cup.
The Red Devils won an epic worldwide match 4-3 but having already qualified, Jose Mourinho can say with some accuracy that his player should not have been involved.
He has won the Premier League, FA Cup, League Cup, Europa League and Community Shield while with United, and the 27-year-old lies one appearance short of 250 for the club.
Belgium announced shortly after seeing Fellaini laid low that he was facing "a couple of weeks" out, with his setback coming at a bad time for United.
"Paul is very important for Manchester United but we also have great players to play instead of him". And Michael Carrick is also not available for the trip to Anfield.
Premier League heavyweights Manchester United are embroiled in a crisis in midfield ahead of their mouthwatering clash against Liverpool this weekend.
The England worldwide then opted for Patrice Evra at left-back, and Paul Scholes, Michael Carrick, Ryan Giggs and Paul Pogba in a midfield four, with Wayne Rooney and Marcus Rashford as his two strikers.