Manchester City striker Valeri Bojinov will be out for at least five months after suffering a knee ligament injury.
Bojinov signed from Fiorentina on 3 August
The 21-year-old made his first start for City in the Manchester derby on Sunday but had to be replaced after just six minutes.
A City statement said Bojinov sustained damage to the lateral and anterior cruciate ligaments of his left knee.
Bojinov signed a four-year contract with City on 3 August after leaving Fiorentina for a reported £6m.
He had made two other substitute appearances for Sven-Goran Erikkson's side before he was picked to start against United.