Lamb also suffered a torn meniscus and a lateral femoral condylar fracture.
Lamb was injured with just under 10 minutes remaining in the second quarter when the veteran went up for a dunk attempt and Raptors guard Terence Davis III contested him at the rim. The contact caused Lamb to fall awkwardly as his left knee bent slightly underneath his body weight.
Despite the injury, Lamb momentarily stayed in the game to shoot and make both free throws following Davis' foul. He then limped off the court and was helped to the locker room.
The 27-year-old averaged 12.5 points, 4.3 rebounds, and 2.1 assists in 28.1 minutes per game this season. He signed a three-year, $31.5-million contract with Indiana last summer after spending four campaigns with the Charlotte Hornets and three with the Oklahoma City Thunder.