Germany goalkeeper Neuer went off at half-time in Bayern's 3-0 home defeat to Borussia Dortmund on Saturday with a calf problem and was replaced by first-team debutant Lukas Raeder.
Although Neuer later claimed he would be fine to face second-tier Kaiserslautern at the Allianz Arena, Guardiola is unsure.
The Spaniard told the press on Tuesday: "He couldn't play if the game was today (Tuesday).
"I'm hoping he'll be able to play tomorrow."
Guardiola did not share Neuer's diagnosis earlier in the week either, admitting that playing the 28-year-old would be "risky".
Bayern are big favourites to win through to the final – which would be their eighth in 12 years – as they go in search of a 17th Pokal success.
That is despite successive league defeats to Augsburg and Dortmund.
Kaiserslautern coach Kosta Runjaic stubbornly declared on Tuesday that his side would "not go to Munich to swap jerseys".
Runjaic has promised an all-action effort from his side, who have already beaten top-flight opposition in Bayer Leverkusen and Hertha Berlin en route to the final.
And Guardiola – who is wary of Kaiserslautern's attacking strengths – feels his players have a point to prove after their recent slump.
"They have three strong strikers," he said.
"We've been analysing things. We're staying relaxed. The players want to win.
"Sometimes a defeat helps you improve. Tomorrow is all about this one game."
