The Argentine was injured during City's 1-0 win over Everton.
But Manchester City boss Manuel Pellegrini admits it is currently 'impossible' to know the duration of Aguero's injury setback.
"It is impossible in this moment to know how long Sergio will be out," Pellegrini said after the Everton game. "He has a problem with the ligament in his knee but tomorrow we will know better how serious it is.
"He will probably miss the game in Rome. We are not sure but it is very difficult when you have a problem with your ligament to recover in three days.
"It is a big problem because Sergio was in a very good moment and he is a decisive player for our team."
Naturally, then, while Manchester City's fans and players were celebrating their 1-0 win against Everton on the one hand, they were hoping for the best with regards to Aguero's injury on the other.
Happy to help my team in this important win. Feliz por ajudar o meu time nesta importante vitória pic.twitter.com/Mmq3I8o5qo— Fernando Oficial (@Fernando_Reges) December 6, 2014
So sorry to see my boy @aguerosergiokun leave the pitch with an injury. We are all hoping it is not too bad.— Yaya Touré (@YayaToure) December 6, 2014