Belgium are fearful that Manchester United midfielder Marouane Fellaini has suffered a knee ligament injury.
Fellaini had to be replaced after 29 minutes of his country's World Cup qualifier away to Bosnia-Herzegovina on Saturday due to a problem with his left leg.
That will have come as a concern to United, who are already without midfielder Paul Pogba due to what Jose Mourinho has described as a "long-term" hamstring injury, while Michael Carrick has been struggling with a knock.
United have a huge away match against Liverpool in the Premier League on October 14, but Belgium boss Roberto Martinez did not sound optimistic about Fellaini's chances.
"We are all worried about Fellaini," Martinez was quoted as saying by Sky Sports.
"We fear that there is ligament damage."
An official from the Royal Belgian Football Association added: "It is something to be worried about on first sight. Ligaments, left knee. More news [on Sunday]."
A resurgent Fellaini has scored three Premier League goals this season, as well as netting in the Champions League defeat of Basel at Old Trafford, after stepping up in the absence of the injured Pogba.
Belgium were 1-0 up at the time of the 29-year-old's substitution, with a thrilling match eventually ending 4-3 in the favour of Martinez's men, who had already booked their place at the tournament in Russia.