Schalke are in talks with Chelsea about taking Baba Rahman on loan for a second successive season.
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.