Gabriel Jesus played his way into the Manchester City team by picking up rhythm for Brazil.
The young striker kept up his record of scoring every time he has started this season when he headed home the opening goal of the Blues' 2-0 win over Crystal Palace.
He scored against Everton before the international break but was then on the bench for games against Dinamo Zagreb and Wolves.
Aguero, who had been carrying an injury, has had the last fortnight to rest up having not been selected for Argentina.
But Jesus, who looked lively for Brazil against Nigeria last week and has stepped up in recent months after struggling last year following the World Cup, was the sharper when the squad reconvened at the Etihad to prepare for Saturday's game.
That swung it for City's manager, who did not need reminding that he has the luxury of two star strikers to be able to choose between as they compete in four competitions.
However many problems they have in defence, both centre-forwards are fit and in form.
"We have two incredible strikers and Sergio is so important for this club," Guardiola said. "Gabriel came back from Brazil, Sergio was 10 days without rhythm in training, that's why I decided. But both are important.
"When Sergio plays he plays a good level and when Gabriel plays he plays a good level. After that I will take a decision . It depends on many, many things."
