Okore, a £4m summer signing from FC Nordsjaelland, has sustained a ruptured anterior cruciate ligament injury in his right knee.
The injury is expected to sideline the 21-year-old for up to nine months.
Paul Lambert feared the worst for the Denmark international after watching him limp off the field half an hour into Villa's 2-1 defeat by Newcastle on Saturday, following a collision with Loic Remy.
A club statement 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."