Aguero, back to full fitness after a calf injury, replaced Alvaro Negredo in the 62nd minute and netted his second goal of the week - he also scored in the FA Cup win over Blackburn in midweek - in the 79th minute to seal City's win at the Etihad.
"We lost Sergio for a month and it was very tough for us because you never want to lose one of the main players but in football sometimes this happens and it was fantastic to see Alvaro Negredo and Edin Dzeko keep scoring goals," said Zabaleta.
Zabaleta is trying to keep his feet firmly on the ground despite talk of City winning the quadruple.
He said: "When you keep on winning games the expectation is really big and we need to keep our feet on the ground.
"We have a chance in all four of the competitions we are in, it's true, but we just need to be focused on what we are doing at the moment.
"We need to continue doing the same thing that we have done so far in the season and then we will see at the end if we win all of them."