Ghana international Rahman spent last season on loan at the Bundesliga side, but has been out of action since he was injured at the African Nations Cup.

The left-back made 21 appearances for the Royal Blues before suffering an anterior cruciate ligament tear.

"Our team doctor met Abdul in London. His knee injury is more complex and [his recovery] might be prolonged," Schalke sporting executive Christian Heidel told Bild.

"We will wait another four weeks and then decide with Baba and his agent whether [another loan] makes sense."

Bild said Schalke had reached an agreement with both Chelsea and the player to extend the loan.