The Spain international, key to the London side's chances against the holders at the Emirates, picked up the knock in Saturday's 1-1 Premier League draw at Birmingham City.
"He didn't appear at training because he's not fit to play. Today he would not be available," Wenger told a news conference on the eve of the first leg.
"I cannot give an answer today. The medical staff say there is still a chance and it will be decided in the warm-up tomorrow night, that's how late we'll leave it.
"Sometimes you don't want to take a gamble and 24 hours more can help. It's important he doesn't get another kick on his knee."
The loss of their captain, who left Barcelona as a teenager seven years ago, would be a huge blow to Arsenal's hopes of building a first-leg lead as his crisp passing and movement are pivotal to their approach.
There was some good news for Wenger, though, as Russian forward Andrei Arshavin trained despite a groin problem while French centre-back William Gallas looks fit to play after overcoming a calf injury more quickly than expected.
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