Rahman is still down the pecking order at Stamford Bridge and Schalke are now in talks to sign the youngster.
The 23-year-old made 21 appearances for the Royal Blues last season before a cruciate ligament injury brought an end to his season.
He also suffered cartilage damage while playing for Ghana at the Africa Cup of Nations in January and he is still on his way back to full fitness.
Fenerbahce have also been linked with a loan move but Heidel insists a second deal is possible.
Heidel told the club's official website: "We are in talks with the player, his agent and Chelsea.
"Ultimately, it is about when Baba will be able to play again. No decision has been made yet."