City have confirmed Bojinov damaged the lateral and anterior cruciate ligaments of his left knee in the 1-0 Premier League defeat of Manchester United.
The 21-year-old was stretchered off during City's derby win at Eastlands after he landed awkwardly following an aerial challenge with Patrice Evra.
A scan has revealed that the Bulgaria international 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.
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