The Clippers, who trail the Suns 2-0 in the series, made the announcement Wednesday. It will mark the Clippers' fifth consecutive game without Leonard in the lineup.
Leonard suffered a right knee sprain in Game 4 of the Clippers' second-round series against the Utah Jazz. Before being sidelined, he was averaging 30.4 points, 7.7 rebounds, 4.4 assists and 2.1 steals over 11 playoff games.
The 29-year-old Leonard hasn't played since that June 14 game against the Jazz because of the sprained right knee.
The Suns, meanwhile, listed veteran point guard Chris Paul as probable for Game 3 of the series. Paul has been in the NBA's health and safety protocols since last week and missed the first two games of the conference finals against Los Angeles.
Before being sidelined, Paul dominated in the Suns' four-game sweep of the Denver Nuggets, averaging 25.5 points on 62.7% shooting while notching 41 assists to just five turnovers in the second-round series.
Game 3 of the Western Conference Finals is scheduled for Thursday night at 9 p.m. EDT at Staples Center in Los Angeles.