Leonard was ruled out before Game 3 with a sprained right knee, which he aggravated at the end of the Clippers' Game 1 victory and played through in their Game 2 loss, according to Lawrence Frank, the Clippers' president of basketball operations.
Leonard's injury is not related to the torn right ACL injury that kept him out all of last season, a source told ESPN's Adrian Wojnarowski.
With Leonard and Paul George (sprained right knee) sidelined, the Clippers put up a fight Thursday night before losing 129-124 to fall behind 2-1 in the first-round series. Devin Booker torched the Clippers for 45 points, and Kevin Durant added 28 points, six rebounds and five assists.
Leonard will be monitored and evaluated daily. He is considered day-to-day, with the Clippers having a quick turnaround before Saturday's 12:30 p.m. PT tipoff for Game 4.
"Symptoms got worse after Game 2," Frank said before Game 3. "Obviously, if we weren't comfortable with [Leonard] playing [in Game 2], we wouldn't have. But after the game, things didn't get better. They got worse. So we will rule him out and kind of take it day by day."
After defeating Phoenix in Game 1, Clippers head coach Ty Lue said his team believed it could win the series. Leonard was unstoppable at times, averaging 34.5 points, 6.5 rebounds, 6.0 assists and 2.0 steals while shooting 54.5% from the field in the first two games, adding to his sterling playoff reputation.
But Lue said he learned of Leonard's knee injury on Wednesday night and was informed before the team's Thursday morning shootaround that he would not have Leonard available.
"It's very deflating," Lue said after the Clippers' loss on Thursday. "I think more so for Kawhi, because you have a guy who's coming off ACL, hasn't missed a rehab session, eats right, eats clean, does everything he can for his body, works extremely hard to get to this point. Then you have something like this happen.
"So it's tough for him, for all the work that he puts in. I feel bad for him."
The Clippers were already without George, who is likely out for the first-round series, sources told Wojnarowski before it started.
Frank said that Leonard's injury will not speed up George's return from his injury, which George suffered on March 21.
"He's made progress," Frank said of George, who has been working out and shooting on the court. "But his recovery time hasn't changed because Kawhi is out tonight. He continues to work his butt off and make progress daily. So day-by-day."
Without Leonard, Norman Powell tried to keep pace with Booker, scoring 42 points. Russell Westbrook had 30 points, 12 assists, eight rebounds and three steals but took the blame for his six turnovers.
Making things even more difficult for the Clippers was the Suns going to the line 46 times compared with the Clippers' 25 attempts.
Westbrook said he felt bad for Leonard not being able to play after putting forth two dominant performances in Phoenix. Leonard played 42 minutes and had 38 points, five rebounds and five assists in a 115-110 win at Phoenix in Game 1.
When asked about the heavy minutes he logged in the victory, Lue said the Clippers carefully worked Leonard's minutes and workload up all season for exactly this reason, saying that it's "what we've been saving up for." Leonard sat out at least one game in back-to-backs until the final two sets of back-to-backs of the regular season.
Leonard played 39 minutes and had 31 points, eight rebounds and seven assists in the Suns' 123-109 win in Game 2 on Tuesday.
"I just feel sorry for him," Westbrook said. "He probably was playing his best basketball in a while, probably the best in the world honestly. It just sucks just for him mentally.
"But it was good to see him be around [at Game 3] and communicate with us on the bench. Hopefully, we can see how he's feeling and see what's next. If he's not back, we have to rally around each other, support each other, find ways to make something happen. It's a series. First to four. Get ready for Saturday afternoon."