Diarra, 32, joined Fulham as in February 2012 after being unattached, but sufferred a knee injury in December and was then released by the club at the end of last season.
However, the former Lyon and Real Madrid man is looking to earn a new deal at Craven Cottage as he continues his recovery.
Still, Jol - who has made seven signings during the transfer window - insists it will be some time before the Mali international is considered for a return to the first-team set-up.
"He is doing a lot of ball work and the warm-ups with us," Jol told The Fulham Chronicle.
"He needs another three or four weeks before playing games with us or the reserves and then we will make a decision on him."
Diarra made 19 appearances during his first spell with Fulham, scoring one goal.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.