Under-pressure Schalke could be without Germany international midfielder Julian Draxler when they take on Bundesliga leaders Bayern Munich at the weekend.
Draxler, 19, returned from Germany's 2-1 friendly win in France suffering from a heavy cold and may join Netherlands striker Klaas-Jan Huntelaar, who has a ruptured blood vessel in his left eye, on the sidelines.
A week after losing 2-1 at home to bottom-of-the-table Greuther Furth, Schalke boss Jens Keller faces the prospect of travelling to Munich without two of his most important players.
"He [Draxler] has not trained today. Let's see if it gets any better tomorrow," Keller said on Thursday.
Schalke will also be without first-choice left back Christian Fuchs after the Austria international was booked for the fifth time this season in last weekend's defeat.
The news caps an unhappy week for Schalke, whose US international Jermaine Jones hit out at fans for cheering their former player Gerald Asamoah and booing current players against Feurth.
Then Draxler used social networks to question his substitution in the same game, but general manager Horst Heldt said: "You can be certain that we will remain calm. We will tackle these issues.
After travelling to Bayern's Allianz Arena, Schalke play at Mainz and then travel to Galatasary in the Champions League.
"We are all judged by the results," Heldt said. "We are facing difficult tasks, but the team has quality."