The Denmark international hobbled off after just 30 minutes of the 2-1 defeat against Newcastle United on Saturday following an innocuous challenge on Loic Remy.
And, after undergoing tests to determine the extent of the problem on Monday, the centre-back has now been told that he has suffered cruciate ligament damage that will require surgery.
A statement from the club on their official website read: "Following clinical examination and the results of an MRI scan, the club can confirm that Jores Okore sustained a ruptured anterior cruciate ligament injury in his right knee in the game against Newcastle on Saturday.
"Jores will undergo surgery in due course and he could be sidelined for a period of up to nine months."
The news comes as a huge blow to Villa boss Paul Lambert, who only signed the player in the close-season.
Okore, 21, arrived on a four-year deal from Nordsjaelland in June for a fee believed to be in the region of £4million and won high praise for his early Premier League performances.
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