Ederson has been ruled out of Manchester City's blockbuster top-of-the-table clash with Liverpool at Anfield on Monday (AEDT).
Ederson was taken off at half-time of City's chaotic 1-1 UEFA Champions League draw with Atalanta on Thursday at San Siro with a "muscular problem".
Substitute goalkeeper Claudio Bravo was later sent off, which left full-back Kyle Walker to take ownership of gloves.
Head coach Guardiola confirmed at a media conference on Friday Ederson is not fit to play but is confident Bravo can fill the sizeable void.
"He's [Ederson] not able to play tomorrow, I don't know [how long he is out]," he said.
"Sunday he cannot play. We have another top keeper.
"Claudio Bravo can do it too, we won the Carabao Cup and FA Cup thanks to Claudio. He has a lot of experience with Chile. I have no doubts."
Bravo endured a difficult first campaign at the Etihad Stadium after signing from Barcelona ahead of the 2016-17 campaign and was replaced by Ederson, who has been a mainstay since joining in June 2017.
The Chilean goalkeeper has not played in a top-flight fixture since May 2018, but Guardiola re-emphasised his support for the veteran.
"Why shouldn't I be confident in my players? I would not like to be a player if my manager doubts me," he added.
"I see him every day in training and how good a goalkeeper he is, we're not going to lose because of Claudio.
"The red card at Atalanta is because we lost the ball in a dangerous position. He won the Copa America, respect that guy, he's an incredible goalkeeper.
"The first season when it was a struggle in a new league is because the team was not at a top level – all the problems were not Claudio.
"It's easy to point the finger. It's the same with Edi, sometimes he makes a good save, sometimes not, why not have confidence?
"All the players in our squad is because we have incredible confidence in them even if they don't play regularly."