The former Real Madrid man was left out of the 3-0 victory over West Bromwich Albion last weekend after he returned from Mali international duty carrying an injury.
After inspecting the problem, Fulham believe it is possible that the midfielder could make it back within a month but, with the potential for further damage to be found after his operation, it is entirely plausible that he could miss a significant chunk of the campaign.
"He has a bit of cartilage floating in his knee", said Martin Jol, speaking ahead of Saturday's game at Wigan Athletic.
"That needs surgery so he is out for three to four weeks, maybe longer. But there could be cruciate ligament damage, and if that is true then it could be three or four months at least."
Though Chris Baird played well in Diarra's role in the Baggies game, this is a huge loss for Jol who has lost various players in central midfield over the summer.