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Aguero was operated on by Guardiola's preferred surgeon, Dr Ramon Cugat, in Barcelona on Thursday (AEST), two days on from aggravating a pre-existing complaint during Tuesday's (AEST) 5-0 win over Burnley.

City have not confirmed the precise nature of the striker's setback but reports in his native Argentina claim Aguero has suffered meniscus damage.

The recovery period from such a surgery typically ranges from four to six weeks, meaning a return for the second leg of City's last-16 encounter against Real Madrid – which will take place on August 7 or 8 – looks a possibility.

Speaking ahead of the trip to Chelsea in the Premier League, Guardiola acknowledged being without his most reliable goalscorer would be a significant setback in the quest for continental glory.

City have won six of the past seven domestic trophies on offer but have not been able to take that dominance onto the European stage, where they are facing a two-season absence pending the outcome of their appeal against a UEFA ban to the Court of Arbitration for Sport.

"Of course, the impact is big," said Guardiola, whose side will progress to a final eight tournament in Lisbon from August 12-23 if they can drive home a 2-1 advantage established at the Santiago Bernabeu.

"To win the Champions League you have to arrive in the right moment, the right time in the best possible condition.

"But at the same time, the situation is what it is and sometimes the rest of the players step forward to solve these situations.

"We have to wait and hopefully [Aguero] can help us against Madrid and, if we are good enough to go through, maybe for the next rounds."

As the games come thick and fast in the restarted season, Guardiola is ready to think creatively with Gabriel Jesus now the only fit natural striker in his first-team squad.

Selecting players in unusual roles to fine effect has become something of a forte for the former Barcelona and Bayern Munich boss, although the suggestion Ilkay Gundogan – who has spent most of this season in defensive midfield – could be used as an attacking spearhead raised eyebrows.

"The players who understand the game and are so smart, they can play in all the positions," Guardiola said. "There are incredible players who play in just one position and Ilkay can play in many positions.

"Not like a striker, a main target, but a striker who can drop and play and get to the box. He is an outstanding player because he is so intelligent.

"That's why, for the disappointing situation for Sergio because we need him a lot, we have Gabriel. We don't have any doubts about him.

"But of course, we play a lot of games in a short time of recovery and preparation. So that's why you have to think of different options."

City produced arguably their two finest performances this season when Guardiola picked teams without a conventional striker.

En route to EFL Cup success, a side with Bernardo Silva and Kevin De Bruyne roaming as nominal leaders of the attack beat Manchester United 3-1 in the first leg of the semi-final – racing into a 3-0 half-time lead.

The same hybrid 4-4-2 setup was also deployed from the start against Madrid, and Guardiola feels he has plenty of options with which to unleash that strategy again.

"[Raheem] Sterling can play there without a doubt, I think it is an incredible position for him," he added.

"At the same time we can play Bernardo. Bernardo played there at Old Trafford and helped us to qualify for the cup final.

"Bernardo is another player like Gundogan who can play in different positions because he is so smart."