England international Walcott will be assessed on Tuesday to see if he is well enough to feature in Arsenal's clash in Germany.
The 23-year-old has been ill overnight and is a doubt for the Group B encounter, which will not see the return of Wojciech Szczesny (ankle) despite the goalkeeper joining in with training on Monday.
Aaron Ramsey misses out after sustaining a groin injury in Saturday's defeat at Manchester United and is joined on the sidelines by Alex Oxlade-Chamberlain (hip), Gervinho (ankle), Kieran Gibbs (thigh), Abou Diaby (thigh) and Tomas Rosicky (hamstring).
Christoph Metzelder suffered an adductor injury on his return from a long-term absence at the end of October and he did not feature in Schalke's weekend defeat at Hoffenheim.
Julian Draxler is in contention to start after recently making his comeback from a broken arm and he was used as a substitute on Saturday.
Lewis Holtby, who has been linked with a move to Arsenal, and reported Liverpool target Klaas-Jan Huntelaar are both expected to be in the first XI.
