Jack was on a fast break when he crumbled to the ground and grabbed his leg in pain. A Nets' trainer came over and the rest of his teammates surrounded him before he was helped to the locker room, without putting any weight on the leg.
The team said he had a sprained knee and wouldn't return.
Earlier in the game, Jack got into a shouting match with Boston's Jae Crowder after Boston forward Jared Sullinger had a hard foul on Brook Lopez. Jack and Crowder had to be separated and were both given technicals.
Jack is averaging 13.2 points and 7.3 assists per game.