Manchester City striker Valeri Bojinov is likely to be out of action for at least three months after injuring his knee during his side's 1-0 victory over Manchester United on Sunday.
The Bulgarian was carried off on a stretcher after falling awkwardly while challenging for a header in the sixth minute of Sunday's derby.
"I was examined by doctors but to be honest I still can't tell you the exact diagnosis because there'll be another medical check-up on Monday," Bojinov told Bulgarian sports Web site www.topsport.bg.
"However, the doctor's initial forecast is that it'll be at least three months out. It wouldn't be a surprise if I have to undergo surgery."
Bojinov joined Manchester City from Italian side Fiorentina only a week before the start of the Premier League season.
City top the league with a maximum nine points.
The 21-year-old will now miss Bulgaria's friendly against Wales on Wednesday. Bulgaria coach Dimitar Penev included Bojinov in his squad as a replacement of injured captain Dimitar Berbatov.
Bojinov will almost certainly miss Bulgaria's Euro 2008 Group G qualifiers against Netherlands and Luxembourg next month.