Along with Gallas (groin), Tomas Rosicky (hamstring) and Alex Song (calf) are all ruled out for the match at West Ham.
Winger Alex Hleb, though, could return from a knee problem to face the Hammers.
"William Gallas is maybe a week or 10 days away, but I would not like to set a time on it really," said Wenger, whose side continue their Champions League campaign against Steaua Bucharest in Romania next week.
"As for Rosicky, you can probably fix the date for Sunderland [next Saturday]. Jens Lehmann will be another week."