Martinez suffered a cruciate ligament tear in his left knee in Bayern's German Super Cup defeat to Borussia Dortmund on Wednesday.
"We need a new player," Guardiola, who before Martinez's injury had said he would make no more signings in this transfer window, told reporters ahead of Sunday's German Cup first round match against Preussen Muenster.
"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," the Spaniard added. "There are still two weeks."
Martinez's absence is a big blow for last season's domestic double winners with the Spain international capable of playing several positions including at the heart of a new-look three-man defence Guardiola fielded against Dortmund.
The Bundesliga starts on Aug 22.