The Spain international suffered knee ligament damage in Wednesday's DFL-Supercup loss to Borussia Dortmund and is now facing a lengthy spell on the sidelines.
Given the versatile Javi Martinez was set to be used predominantly at the back this season, Guardiola believes it imperative that Bayern now attempt to sign a new centre-half by the end of the month.
"He was in outstanding form," the Catalan coach told Bild. "We will miss him very much.
"We certainly need a new player. We still have 15 days and will respond."
Guardiola confirmed that he has identified several possible replacements but he refused to name names.
"We are looking for a young player," he stated. "He shall have to be big, at least 1.95 meters tall, fast, strong in the air.
"He should also be good with both his right and left foot, and flexible ... And cost little!
"But we are perhaps a little late."