The Mali international has been injured since December and although the Dutchman backs him for a return before the end of the season, the Fulham boss admitted he suffered a setback
The 31-year-old has been restricted to just eight appearances for the Cottagers this season after picking up a knee problem whilst on international duty.
The former Lyon and Real Madrid man has not featured since the beginning of December and was also forced to miss the Africa Cup of Nations in January, leaving Jol bemoaning his fitness problems.
"Diarra is still not right," Jol told the Fulham Chronicle. "He will need quite a while to come back, hopefully before the end of the season.
"He had an operation, looked better and then had a rest and it was even more painful.
"Hopefully in the next couple of weeks we will know more."
Diarra, who made 11 appearances last season after joining on a free transfer from Monaco in February, has four months remaining on his Fulham contract.