The goalkeeper, who has made a series of high-profile errors in the opening weeks of the season, aggravated an existing problem in Germany's 2-1 victory over England at Wembley.
Lehmann will also miss the second leg of the Gunners' Champions League qualifier against Sparta Prague at Emirates Stadium and the home clash with Portsmouth on September 2.
Wenger said: "Jens is out and he will miss two weeks, I think. Maybe more. I don't know.
"He is going to see a specialist in Germany. He had this injury before but it has got much worse.
"Apart from the mistakes, he's had good games," said the Frenchman. "You have to defend your goalkeeper and your players.
"You don't judge it on how many mistakes they make - you have confidence in your goalkeeper.
"The problem when you get older is people say you're in decline, but I am still convinced Jens is top drawer.
"Almunia is equipped to take responsibility because he did very, very well when he played last year.
"It was very reassuring.
"He is mature, he has all the attributes you need to be successful. It was a close decision who is my number-one keeper because of his performances last season."