Guardiola had hoped Aguero would be back in training ahead of the trip to Portugal, but says he is not worried about his striker’s fitness.
“I am not concerned,” he said at his pre-match press conference.
“He didn't train. He has some disturbance in his knee, nothing serious, but he didn’t train.”
City go into our penultimate group game having already guaranteed our place in the knockout stage of the Champions League and travel to the Estádio do Dragão looking to secure top spot in Group C.
It is the first of nine matches in a busy December and Guardiola admitted he is thinking of making changes against the Portuguese champions, though he has ruled out including youngsters from the Academy in his squad.
“I am thinking about it,” he added when asked if he would rotate.
“We have a lot of games. But it’s not rotation. Every game is played because the best player is selected for the specific game.”
He added: “No [Academy players won’t be used].
“We have 19 players fit for the first team squad and all will travel to Porto. It’s a difficult opponent. We need the best players as possible.”
Get three months free subscription to CITY+ with Manchester City's Black Friday deals. Use 90DaysFree at checkout by midnight on Monday 30 November.
After your three-month free period, CITY+ costs £1.99 a month.