Spurs take a 1-0 aggregate lead into the game at the Etihad, meaning City need to win by two clear goals to progress to the semi-finals.
Aguero returned from injury in the first leg after missing Citys previous two fixtures following his decision to come off in their 2-0 win against Fulham on March 30th.
Guardiola refused to describe Agueros issue as an injury, revealing instead that he felt something and didnt want to aggravate it by staying on at Craven Cottage.
The Argentine was given ten days to recover but he looked far from himself when he returned against Spurs and saw his penalty saved by Hugo Lloris after ten minutes in north London.
City hoped Aguero could play his way through the issue but his performance against Crystal Palace on Sunday has exacerbated the problem and left Guardiola pondering his options.
City fear that, while Aguero is not strictly injured, hes playing within himself and not able to push himself to the limits at a crucial stage of the season.
Pep Guardiola on Sergio Aguero's fitness
Hes fit, he needs games to get his best condition. Other players, like Bernardo and the other strikers, they are fit and firing. Tomorrow is not a lot about physical condition, do they want to get through. This is the moment. The desire to get through, the illusion, what is the pressure. If we lose, then we have failed and next year well be there again in the Champions LeagRead More – Source