Bravo is facing a lengthy lay-off after rupturing an Achilles tendon in training.
The only other back-up City have to number one Ederson is 20-year-old local Academy product Daniel Grimshaw. Neither Muric nor Grimshaw have made a first-team appearance for City.
City have been linked with Diego Cavalieri, the former Liverpool reserve who is now a free agent after leaving Crystal Palace, but Guardiola said that is not being considered.
The City boss said: “From then beginning it was Aro, Aro – that was the only option.
“The transfer window is closed so we cannot buy. With players who have retired – maybe – but when we knew it was possible for Aro to come back the decision was quick.”
Muric is likely to go straight onto the bench as the champions face Wolves on Saturday.
“We know him,” said Guardiola. “Last season he trained with us all season and he knows the way we work. We loaned him because the second team is not a real competition. We wanted to see him play every weekend.”
City are still unsure how long Chile international Bravo will be out for but the 35-year-old has undergone successful surgery in Barcelona.
Guardiola said: “I don’t know – but the surgery was good and we’ll see.”
Bravo himself has tweeted some pictures of his foot in a bandage following his operation.
“Thank you so much affection and concern,” he wrote.
Buenos días a todos, muchas gracias por tanto cariño y preocupación, en forma especial a los niños y funcionarios de @teleton ❤️Aquí estoy recuperándome gracias a todas las muestras de esfuerzo y sacrificio que me demuestran día a día.#VamosMuchachosQueSePuede #NoRendirse💪🏻 pic.twitter.com/00WjJ8aUGq— Claudio Bravo Muñoz (@C1audioBravo) August 24, 2018
The injury to Bravo was the second such blow City suffered in the space of a week after playmaker Kevin De Bruyne sustained knee ligament damage.
Guardiola feels the lack of pre-season training some players had this summer as a result of the World Cup is affecting his squad.
Speaking at a press conference, Guardiola said: “We are not far away but we’re still not fit.
“We see how much time the players need for regeneration after games. Normally it is quicker. They need more time.
“This is because we didn’t make a pre-season. After three or four days we played the Community Shield. A few players are much better but others need more time to win power and strength.”
City captain Vincent Kompany this week celebrated the 10th anniversary of his arrival at the club and he is set to make his 250th Premier League appearance against Wolves at Molineux.
Guardiola said: “Happy anniversary. It’s a long time, it’s many games. Unfortunately in the last years it wasn’t more.
“I’ve no doubts about his quality. When he is fit he is an excellent central defender. He’s a big personality and he’s loved in the locker room.”
City were taken to a penalty shoot-out by then Championship Wolves in the Carabao Cup last season.
Guardiola said: “They destroyed the Championship last season. They had an incredible season and they play in an attractive way. We know from the beginning how tough it will be.”
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