It was unclear when Leonard injured the knee or when it became apparent for the Raptors.
Leonard, averaging 27 points and 7.7 rebounds, has been held out to rest periodically throughout the season as 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 earlier ruled out for at least three weeks with a left thumb injury.
The Nets gave the Raptors a tough time Monday, with Leonard hitting a game-winning bank shot with 4.4 seconds left. He had 30 points for the game.
Leonard has been in slump by his standards, but Monday he also matched a career-best eight assists.
The Raptors (42-16) have won five in a row to get back within a game of the Milwaukee Bucks for the top spot in the Eastern Conference.
Information from The Associated Press was used in this report.