Bastian Schweinsteiger is a doubt for the DFB Pokal final against Borussia Dortmund after limping out of Bayern Munich's final Bundesliga match of the season after 37 minutes.
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Schweinsteiger, 29, could also face a race against time to prove his fitness ahead of the World Cup with Germany meeting in Berlin to begin their preparations for the tournament shortly after the final.
"Bastian told me that he has problems with his left knee," Guardiola said in a postmatch news conference. "The doctor told me that it is the patella tendon. We still have a few days, and I have hope."
Schweinsteiger is now set for a MRI scan, with local Munich paper Muenchener Merkur reporting that he was one of the first to leave Bayern's title celebrations at Munich's Postpalast on Saturday.
"I am not a doctor, and I can't say anything about it. But when you are not fit, you are never in a good mood," Bayern captain Philipp Lahm was quoted as saying by the paper.
Schweinsteiger had missed a large part of the season following two ankle surgeries in 2013.
His role at Bayern has been questioned by the German public, and critics have said that the midfielder is too slow for the Guardiola system and has passed his prime.
Elsewhere, Bayern Munich CEO Karl-Heinz Rummenigge has targeted a domestic double ahead of the showdown with Dortmund, but Guardiola has not echoed his confidence.
"It'll be tough against Borussia Dortmund, but I'm looking forward to the match," Rummenigge told the club's official website. "It would be wonderful if we can do it [win the double]. It would turn a good season into a very, very good one. And it would be good for the Bavarian soul."
But Guardiola, who came under fire for the manner of Bayern's Champions League exit against Real Madrid, has warned against over-confidence.
"It's going to be a difficult summer, should we not win [the cup]," Guardiola said. "But I will stick it out without any problems. The goal then would be to plan the squad for the next season to make the team better."