Diarra has left Lokomotiv Moscow following a dispute with coach Leonid Kuchuk and is now set to link up with former boss Harry Redknapp, who he played under at Portsmouth.
The 29-year-old French midfielder, who has also played for Chelsea and Arsenal, arrived in London on Friday and is expected to sign for Rangers as long as there are no problems with the medical.
Redknapp signed Diarra from Arsenal in January 2008, for a fee of around £5.5million, and sold the Frenchman to Real Madrid at a huge profit a year later.
Speaking after the 1-0 victory over Sunderland at Loftus Road on Saturday, Redknapp said: "He's signed. I think everything is agreed. He was down with the chairman last night and I am just delighted to have him.
"I signed him at Portsmouth and he was a fantastic player, when I left they sold him to Real Madrid for £22million.
"He's a top, top, top player."