It was unclear whenever it became apparent or if Leonard wounded the knee.
Leonard, averaging 27 points and 7.7 rebounds, was held out to rest periodically through the entire season since the Raptors carefully manage his minutes after he missed most of last season with a quad injury.
The Raptors will also be without guard Fred VanVleet, who was ruled out for three or more weeks with a left thumb injury.
The Nets gave the Raptors a tough time Monday, with Leonard hitting a game-winning bank taken with 4.4 seconds left. He had 30 points for the match.
Leonard was by his standards doing slump, but Monday he matched a career-best eight assists.
The Raptors (42-16) have won five in a row to get back within a match of their Milwaukee Bucks for the top spot in the Eastern Conference.