With the Premier League champions unable to recruit a goalkeeper registered with another club until January, they were left with the option of signing a free agent.
However, Man City would still have needed permission from the Premier League to register a free agent to play, and there is no guarantee that it would have been given.
When asked by The Telegraph about whether the rules need to be reviewed, Guardiola replied: "Yes. Not just for goalkeepers but for when there are big injuries to players. If we had not been allowed to bring back Aro, I don't know what would have happened.
"But the rules are the rules. Maybe the federation can think about that when there are long injuries and it is still the period where the European window is open. Then maybe you could do it.
"Here it is closed, and it is good that the competition starts with the whole squad [in place]. But maybe we could think about that, especially when the injuries are five or six months."
City, who have been linked with former Liverpool and Crystal Palace stopper Diego Cavaleri in the past few days. allowed Joe Hart to join Burnley earlier this month having already sold Angus Gunn to Southampton.