Jose Mourinho could be without Marouane Fellaini for Saturday's visit to Liverpool and possibly for weeks after the Manchester United midfielder suffered suspected knee ligament damage on global duty with Belgium.
He was treated briefly by medical staff and appeared to indicate there was a problem in his left knee, before being replaced by Anderlecht's Leander Dendocker.
Marouane Fellaini was forced off after 29 minutes in Belgium's WCQ clash at Bosnia-Herzegovina with an apparent knee injury.
"We fear that there is ligament damage".
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And after the game, Martinez said: "We are all anxious about Fellaini".
Jose Mourinho will be hoping the injury is nothing too serious, with the Belgian starting this season in red hot form, scoring four goals already.
Fellaini has scored four goals and provided one assist this season already for Manchester United, who return to action after the global break with a trip to Liverpool in the Premier League next Saturday.
The Belgian scored twice against Crystal Palace in the final match before the global break and has been preferred in midfield ahead of Ander Herrera, the club's player of the year last season.