Rahman, 23, got injured on International duty at the Africa Cup of Nations in January and has since not fully recovered. The Ghanaian was loaned out to Schalke from Chelsea last season.
The Ghana international is a confirmed target for Schalke 04 chief Christian Heidel, though he is still recovering from a knee injury.
Schalke have now asked Chelsea for a medical update before making their next move.
Meanwhile reports in Germany indicates Schalke are prepared to wait until January to re-sign Baba after he impressed everyone at the club last season.