A day after Arsenal revealed captain Cesc Fabregas's season was ended by a broken leg, the Premier League side's title tilt took a fresh hit after Gallas was sidelined with a calf injury that has troubled him since February.
"Gallas is (out for) longer than three weeks, I would say personally five weeks for Gallas," Wenger told reporters, effectively ruling him out of the domestic season.
Gallas was one of five players injured in Tuesday's 2-2 Champions League quarter-final first-leg draw with Barcelona on what could prove to be a damaging night for the London club.
Defender Gael Clichy and midfielder Denilson also picked up knocks and face late fitness tests ahead of Arsenal's league match at home to Wolverhampton Wanderers on Saturday.
Wenger's third-placed side trail league leaders Manchester United by four points with six matches remaining and his wounded squad also face a Champions League second leg trip to the Nou Camp as they bid to reach a second final in four years.
"We want to focus on our targets and still achieve them," Wenger said.
"I think I have torn it (calf muscle). If that's true, then I'll be out for a long time, at best after three weeks but at worst I won't play at all this season."
Wenger was more optimistic of the Russian international's chances.
"21 days minimum, by April 23 he should be clear to play again."
"We believe that it (the leg break) is from the kick from Puyol which led to the penalty," Wenger said before adding that Fabregas had undergone a scan before the match kicked off.
"There is some suggestion that he played with a broken bone already, that is not true... a bruised bone, but not a broken one before the game."
