The Pacers announced Monday that Lamb has a torn ACL, a torn lateral meniscus and a fracture in his left knee.
Lamb suffered the injury in Sunday's loss to the Toronto Raptors. Initial X-rays on his knee were negative, and Lamb told reporters after the game that he hoped the injury wasn't "too serious."
The Pacers have not determined a specific date for Lamb's surgery.
Lamb, 27, averaged 12.5 points and 4.3 rebounds in 46 games this season, his first with the Pacers.