Leonard, who watched Game 3 from a suite at Staples Center, suffered the sprained knee in Game 4 of the Clippers' second-round series against the Utah Jazz. Before being relegated to the bench due to the injury, 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 knee ailment.
Meanwhile, Clippers forward Marcus Morris Sr. is expected to be a game-time decision for Game 4 against Phoenix.
Lue said the team didn't know whether Morris would play in Game 3 until shortly before tipoff, leading to the decision to start second-year guard Terance Mann in the Clippers' 106-92 win over the Suns on Thursday.
Morris has been limited in the conference finals because of a left knee injury. The veteran forward played 24 minutes and recorded eight points and five rebounds in Game 3.
Game 4 of the best-of-seven series is set for Saturday at 9 p.m. EDT in Los Angeles. The Suns hold a 2-1 lead in the series.