Aguero, 32, was withdrawn at half-time in City’s 1-1 draw with West Ham due to a muscular injury, handing Guardiola a fresh injury blow.
It was just the Argentina international’s third game back after a knee operation in June but his latest injury has left City short in the striking department.
With fellow frontman Gabriel Jesus already out of action, City have travelled to Marseille for their second Champions League group stage game without a recognised striker.
Raheem Sterling and Phil Foden have both deputised in the role in the past but Guardiola will hope Aguero can get back to full fitness as soon as possible.
However, he admitted it could take a month to have the striker back in his starting XI.
‘It depends on the injury,’ Guardiola said. ‘When it is minimum it will be 10-15 days. If it is longer it will be three weeks to a month. I don’t know.’
Former Barcelona and Bayern Munich manager Guardiola admitted he had considered signing a new striker this summer, but there were insufficient funds remaining after spending big on centre-backs Nathan Ake and Ruben Dias.
‘We considered it, we thought about it another type of striker but we could not,’ he added.
‘If you decide to buy a striker, it has to be a striker in the league of Gabriel and Sergio, but we cannot afford it. That is the reality.’
Aguero and Jesus are not the only players out of action for City on Tuesday night.
‘We are not an exception compared to the other teams in Europe,’ said Guardiola. ‘We’re on the same schedule. Now it is recovery, preparation, recovery and preparation, recovery and preparation.
‘We don’t have time to practise much. Everyone got the statistics – in the Premier League, we have 47 per cent more muscular injuries than the same period last season. It’s not just about City, it’s about everyone.
‘If the people can reflect on this and the guys to protect the players it will be good for the future, if we don’t it will increase. Now we have to protect the players as best as possible. The strongest ones will survive, the others will fall down.’