Marouane Fellaini looks set for a spell on the sidelines after Jose Mourinho revealed the midfielder suffered a "bad" back injury in Manchester United's 1-0 win at Hull City.
Though Fellani played all 90 minutes, United's medical staff discovered he had a back injury after the game, which United won thanks to Marcus Rashford's injury-time winner.
"I am going to lose Fellaini for a while. He has a muscle injury in his back and it is not good," said Mourinho.
Mourinho was then asked how long he thought Fellaini may be out and he said: "I don't like to say that but it is bad."
Fellaini has established as a regular under Mourinho, starting alongside Michael Carrick in the Community Shield win over Leicester and alongside Ander Herrera in United's win over Bournemouth before being picked to partner Paul Pogba in the wins over Southampton and Hull.
The Belgian international's absence will offer an opportunity for Carrick, Herrera and Morgan Schneiderlin to break into the team alongside Pogba.
Fellaini was due to link up with the Belgian squad for their friendly against Spain on Thursday in Brussels and their World Cup qualifier against Cyprus on September 6, which is live on Sky Sports 3 HD.
Despite his injury, Mourinho expects Fellaini to travel so the Belgian medical staff can assess him for themselves: "I think he has to go. They have to look at him but he has to travel."