Garnett, who has been held out of basketball activities for the past two days due to a left adductor strain, is now also battling the flu and that, Rivers said, more than the injury is what will keep him off the court against the rival Heat.
Jeff Green started in Garnett's place and scored a career-high 43 points in the loss.
"[Garnett is] sick, really. It's not the injury," Rivers said. "He just has a terrible flu or whatever and, with the injury, we just felt the combination -- it was too much to play."
Garnett didn't go through the team's shootaround on Monday morning, but was still listed as a game-time decision. Rivers got a look at him when he arrived at the arena and determined he wasn't fit for action.
"He's in [the trainer's room] now. Right when I looked at him, I said, 'You're not playing,' " said Rivers. "He wanted to play, and even [team trainer] Eddie [Lacerte] came to get me to tell him not to play."
Would Garnett have played if not for the illness?
"Yeah, he would have played," Rivers admitted.
Even though Rivers has preached health and rest over wins and playoff seeding, he said the team would have preferred to have Garnett on the floor for Monday's game.
"You want Kevin on the floor," Rivers said. "I guess it gives him more rest, and maybe that will help too."
Rivers said he expects Garnett to be on the team's flight Tuesday afternoon for the start of a three-game road trip in New Orleans on Wednesday.