Leonard, a first-team All-NBA selection this season, has been sidelined with since aggravating a left ankle injury in the third quarter of Game 1 of the series.
The Spurs led by 23 points at the time of Leonard's injury. They lost that game as well as the next two games, both of which Leonard missed.
San Antonio trails the best-of-seven series 3-0.
Leonard originally injured his ankle in Game 5 of the second-round playoff series against the Houston Rockets. He re-injured it when he landed on the foot of Warriors center Zaza Pachulia.
The Spurs also ruled center/forward David Lee out on Monday due to a left knee injury.
Warriors forward Andre Iguodala, who has been bothered by knee soreness, was cleared to play Monday. He saw limited action Saturday in Game 3.