The striker is set to miss a third consecutive Premier League game after first going into isolation on January 10.
Speaking at a news conference ahead of the Villa game, City boss Pep Guardiola said: "He is not ready for tomorrow. He's still isolated.
"We need Aguero, we miss him.
"The numbers speak for itself - he's one of the best strikers in our history. The player can create for himself and win the games by himself.
"From a long time, he is not with us. We could start complaining but we did completely the opposite. We wait for him.
Guardiola says City have missed Aguero's ability to create chances for himself
"We want him back as soon as possible but at the same time, we have to move forward and with the quality of players that we have, we can do it and find alternatives."