City recovered from 2-0 down to beat the Hornets 4-2 in their fourth-round tie at the Etihad Stadium thanks to a second-half Aguero.
City manager Manuel Pellegrini said: "Sergio is a top player. The first idea was for him not to play the 90 minutes today.
"The idea was for him to play 60 or 70 minutes but I think he made the difference."
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