Sergio Aguero is set to miss four to six weeks of action as he deals with a knee ligament injury, Manchester City coach Manuel Pellegrini said on Tuesday.
Aguero was injured on Saturday in the second minute of Manchester City's 1-0 win over Everton.
City had already ruled their 19-goal top scorer out of Wednesday's Champions League decider against Roma, and Pellegrini revealed the time frame for the Argentine's return at Tuesday's news conference in Italy.
"It is difficult to know exactly how long he will be injured. It could be four, five or six weeks. It depends a lot on the way he recovers," Pellegrini said.
The manager also confirmed that his striker had undergone a scan.
"We know he is injured. He cannot play tomorrow. He will see how long he will be out, but I think it is more important for us to think about the players we have available for an important game against Roma."
Aguero said there was a "strange" feeling in his knee when he came off in tears.
Aguero scored a hat-trick in City's last Champions League game, the 3-2 win over Bayern Munich, and has delivered five goals in his last three European matches.
City are already without the suspended Yaya Toure against Roma but received fitness boosts for David Silva and Vincent Kompany, who both trained on Tuesday.
Aguero would also miss fixtures against Leicester, Crystal Palace, West Brom and Burnley this month, the New Year's Day clash with Sunderland and an FA Cup third-round tie against Sheffield Wednesday.
Should he be ruled out for six weeks, the 26-year-old would also miss tough games against Everton and Arsenal in early January, but he could be back to face title rivals Chelsea on the last day of the month.