Wenger told the club's official website that the problem should not be a long-term issue but he will be cautious with Walcott's recovery.
He said: "Theo is out for Saturday, for sure. He has an injury to his chest and lungs.
"We have to treat that clinically. He's quickly short of breath when he makes an effort but it's bruising and that will heal.
"It's no major concern but it will take we think about two weeks to be back to a normal level."
The 23-year-old will definitely miss Premier League matches against Norwich City and Queens Park Rangers plus a Champions League clash with Schalke.