The Sweden international, 21, came off early during the Cottagers' 1-0 pre-season friendly defeat at Werder Bremen on Sunday night and will be in a race against time to be fit for the season opener away at Sunderland on August 17.
"Hopefully it is not too bad but he may be out for a couple of weeks," Jol told reporters after the loss in Germany.
"That is not what you want from pre-season. Kaca has made a lot of big steps and progress with us."
Fulham, under new ownership following Shahid Khan's takeover of the club, have added five new faces to their ranks over the summer, but Jol is still targeting more acquisitions.
Sascha Riether, Fernando Amorebieta, Derek Boateng, Maarten Stekelenburg and Ange-Freddy Plumain have all moved to Craven Cottage but a striker remains Jol's priority.
"We will try to get someone up front," he said.
"We need a few more goals, since Clint (Dempsey) left we do not have enough goals in us, so we are working every day to get someone.
"We have some high-quality players in consideration and we would like another midfielder as well."
Source: PA
Source: PA