Draxler could make Schalke return
Germany attacking midfielder Draxler has been out of action since December because of an ankle injury.
However, the 20-year-old has now returned to training and Keller is hopeful the reported Arsenal and Real Madrid target could be involved as Schalke look to boost their UEFA Champions League ambitions.
"He is certainly not fit enough to start," Keller said. "But I would like him in the squad."
Defender and captain Benedikt Howedes has recovered quicker than expected from a thigh problem, but the 25-year-old is still a doubt for the trip to the BayArena.
"The healing process (for Howedes) has been very quick. But he still had a little trouble with some movements and is still questionable," Keller added.
The news is not as positive for Atsuto Uchida, who tore a thigh muscle against Hannover last weekend. Keller confirmed that the full-back "will likely need surgery."
Schalke sit fourth in the Bundesliga ahead of the clash with Leverkusen and could potentially leapfrog local rivals Borussia Dortmund, who sit third, with a win.