Fulham manager Martin Jol says midfielder Mahamadou Diarra could miss the majority of the season if his knee injury is deemed serious.
The 31-year-old picked up the injury while on international duty with Mali in their African Cup of Nations qualifier against Botswana, and there are fears he may have damaged his antertior cruciate ligament.
Diarra was making his comeback for his country after he went a year without being selected for international duty as a result of not having a club.
The injury forced him to miss Fulham's 2-0 win over West Brom last week, and now it appears the setback could be more severe than first feared.
Jol told reporters: "There's a little piece of cartilage floating in his knee, so we have to operate that in any case, and that can be three or four weeks.
"But if there is damage to his cruciate ligament, for example, it will take a lot longer. It will take him three, four, five months at least."
Fulham travel to Wigan in the Premier League on Saturday.