Guardiola could also start Leroy Sane, who suffered an ankle injury in last season's FA Cup win over Cardiff after a awful tackle from Joe Bennett.
Aguero is City's all-time leading scorer with 204 goals, and has scored three times in five Premier League games this season. City, who are also without Kevin de Bruyne (knee), will be anxious to get back to winning ways after the hiccup against Lyon and Guardiola is expecting a physical challenge against Neil Warnock's promoted Cardiff side.
The Spaniard said: "I don't know [how long]".
Guardiola has been effusive in his praise for Aguero over the last 12 months and claimed his players had no reason to apologise for Wednesday's shock 2-1 defeat by Lyon.
Britain’s May Says Brexit Negotiations Have Reached ‘an Impasse’
They want a further swing of the momentum away from the British demos and back to the Brussels oligarchy. Brexit supporters say such fears are exaggerated and Britain would thrive in the long term.
"It's always hard. Last season we did nearly everything well", he said. "You have to be able to handle it".
"I understand when you take that decision - it happened in Barcelona many times - I always know the reaction [will be bad] when the result is not good, but I have to take decisions".
"Players like him, Vincent [Kompany] and David [Silva] show their commitment to the club every day in training".
"It's a new game and that's all", Guardiola said, when asked if he was concerned about a repeat performance.
He fired back: 'I'm not going to play that game. Third-placed City's most recent Premier League trip to Cardiff came in 2013 when they lost 3-2. (City) are a unique team. I'm very happy about everything that involves the Club, my teammates, the staff and the management team.