Rahman, 23, made 21 appearances for Schalke last season before his campaign was cut short by a cruciate ligament injury while in action for Ghana at the African Nations Cup in January.

And Heidel said any move could depend on when Rahman is fit to return to action.

"We are in talks with the player, his agent and Chelsea," Heidel told Schalke's official website.

"Ultimately, it is about when Baba will be able to play again. No decision has been made yet."

Heidel added that Schalke have not lined up any other new signings but said that could change.

"Nothing is planned, but you should never say never in football," he said.

"I do not know what will happen tomorrow, the day after or next week. The fact is a transfer is possible until Aug. 31, but our squad is very good in terms of quality and depth."