The 31-year-old picked up the injury whilst on international duty with Mali in their African Cup of Nations qualifier against Botswana.
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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 the Cottagers' 2-0 win over West Brom last week, and now it appears the injury could be more severe than first feared.
Jol revealed Diarra could require surgery to fix the problem, which could see him ruled out for a minimum of three weeks - but if it emerges the midfielder has damaged his cruciate ligament, he could be out for up to five months.
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."