The Spaniard damaged a cruciate knee ligament as Bayern lost out to Borussia Dortmund 2-0 in the DFL-Supercup on Wednesday, with the Munich boss now stating that he needs to make a new signing immediately.
"We need a new player," Guardiola told reporters at a press conference.
"We have to react and there are candidates. There are many candidates but only a few who would do for us. With this injury we have a special situation and now we must act. There are still two weeks."
Bayern get their Bundlesiga campaign underway next weekend when they welcome Wolfsburg to Munich.