The French international sustained the injury in the final training session for Tuesday's UEFA Champions League game at home to Hamburg.

"We've lost Gallas in the final shot of training," Wenger told a press conference. "He pulled his thigh and will be out for a few weeks."

Wenger rued his luck after losing the seasoned defender, yet kept looking on the bright side.

"It is a blow because he stabilizes our defence. As well, it's a good opportunity for a young player to come and show that he's good.

"But of course, overall, we wanted him to be fit -- not only because of his quality but as well because of the number of games we have coming up...we are playing three times a week."

Gallas was brought in to fill the void left by defenders who sustained long term injuries.

Arsenal are currently in joint second place with FC Porto in their Champions League Group G with 7 points.