Guardiola was unsure how long the player would be absent for and if he would be available for Wednesday's Premier League clash against Brighton, even if he returned a negative test.

The City boss said: "Unfortunately he was in contact, more than usual, with one person who is positive. That is why he must isolate for a few days.

"I don't know (how long for) exactly because, like all of us, he has been tested six times in last 10-15 days.

"He is negative. Today he is negative too but the protocol says in some cases they must isolate. In others we are still in touch and playing football.

"Unfortunately he could not play today."

Aguero's presence was hardly required as City eased to a 3-0 victory over their Championship opponents with an early double from Bernardo Silva and a further strike from Phil Foden. All three goals came inside the first 33 minutes and effectively ended the game as a contest.

Guardiola said: "Yes the first objective was done – qualify for next round. We did it with a really good first half.

"We made nice goals. I am happy for Bernardo. He made two goals and could have scored more. The first half was really good.

"In the second half they changed the set-up. We struggled a little bit to find our spaces but we are in the next round."

The victory came four days after City, the 2019 FA Cup winners, reached the Carabao Cup final for a fourth successive year.

Guardiola said: "If you follow the trajectory for our team, we don't drop one competition. That is why we are in finals four times in a row in the Carabao Cup and why we won the FA Cup two seasons ago."

It was only when City eased off in the second half that Birmingham began to create any openings.

Substitute Jeremie Bela twice tested goalkeeper Zack Steffen and shot narrowly over in the latter stages but prior to that the visitors were unable to make an impression.

Manager Aitor Karanka, whose side have now lost six of their last seven games, said: "We finished the game in the way we should have started.

"We were not as aggressive as we should have been but in the second half everything changed. We approached it much better. I wanted to play 90 minutes in the way we played the last 45.

"The main thing when you are coming to this kind of game is to be aggressive and strong. If you allow them to play, if you are jogging behind the ball instead of being aggressive, the only thing that can happen is what happened.

"I was pleased when I saw their first XI because it meant Pep respected us. We wanted to do something here. We showed the second half we should have been more aggressive from the beginning."