On the verge of returning to the pitch after an injury layoff since January and with the Blues holding off moves away until the winter, the 23-year-old was widely tipped to make the cut for the competitions, especially the topflight.
However, the Ghanaian has been snubbed on the two rosters, casting doubts over his importance at Stamford Bridge.
The situation looks more worrying considering the Blues are currently short on options in his position, having failed to land Juventus star Alex Sandro as cover for first-choice Marcos Alonso.
Chelsea had space to register the Ghana international but, somehow, chose not to list him for the topflight.
That said, Baba will be eligible to feature in cup competitions and is very much expected to have a run out as part his rehabilitation plan.
Schalke were keen on reacquiring the former Augsburg man on loan but Chelsea, who boast of one of the best medical facilities, want to keep a close eye on him until January, effectively stalling the move.
Goal understands the Royal Blues remain Baba's likeliest destination in the winter, having played on loan at the VELTINS-Arena last campaign.
Galatasaray are also said to have expressed interest but a deal is unlikely although the Turkish transfer window is still open.
On his road to recovery, Baba is expected to play at least three games with the Blues' under-23 side, as did Kurt Zouma and Marco van Ginkel last season.