Manchester City have confirmed that Valeri Bojinov will be sidelined for at least five months.
Bojinov was stretchered off during City's derby win over Manchester United on Sunday after he landed awkwardly following an aerial challenge with Patrice Evra.
A scan has revealed that the 21-year-old has damaged lateral and anterior ligaments in his knee.
City admit that the player, who arrived from Fiorentina earlier this month, will be missing until January at the earliest.
Bojinov revealed he will have an operation in Italy later this week.
"I will have surgery in Rome," Bojinov told Darik radio.
"It will be done by the doctor who operated on Francesco Totti. He [the doctor] is a friend of my agent and we decided to go with him."