The 32-year-old, who came to the end of his contract at Craven Cottage in the summer, suffered the problem during a match in the reserve team.
"Grygera has had a setback because he got injured," the Daily Star quotes Jol as saying. "He looked ever so well, played in the reserves and kept them together.
"He has a thigh problem so we have to wait and see because we wanted to make a decision on him next week and now there is a little step back again."
Jol has been open about his uncertainty over offering the defender a new deal.