The Cottagers allowed the midfielder's contract to run down in 2012/13, with a lengthy spell on the sidelines keeping him out of their plans.
Diarra has not featured since December, with a knee complaint forcing him under the knife.
It has been a long road back for the former Real Madrid man, but there is now light at the end of the tunnel.
The 32-year-old has been training with Fulham throughout the summer and the Premier League outfit are prepared to offer him fresh terms if he can prove his fitness.
Asked to offer an update on that process, Cottagers boss Martin Jol said in the Chronicle: "Diarra is coming back, he's probably at 50 per cent.
"He's not really fit but if there's no contact, like in possession games, then he can join us in training.
"He's not quite there where we want him to be yet."