The 22-year-old joined Chelsea in 2015 from Augsburg for a fee in the region of £14million, but struggled to break into the first-team picture.
Schalke offered the full-back the opportunity to get his career back on track last season with a loan spell at the Veltins-Arena.
Rahman impressed during his time in Gelsenkirchen, but missed the second half of the season with a knee injury sustained at the Africa Cup of Nations with Ghana.
The Bundesliga outfit were expected to make his switch permanent this summer, but Heidel insists they will now wait to see how his injury progresses.
Heidel said: “Our team doctor met with Baba in London. His knee injury is much more complex and lengthy than anticipated.
“We will wait until July and then decide with Baba and his agent whether it makes sense to open negotiations with Chelsea.”