Garnett is battling a left adductor strain that flared on Friday and kept him out of Saturday's win over the Charlotte Bobcats. Rivers said the team will reevaluate Garnett on Monday depending on what he's able to do leading up to the game.
"We don’t know what he’s going to do [Monday]," Rivers said. "My guess is he’s 50/50 at best."
Asked if there was any long-term concern with Garnett, Rivers downplayed the injury.
"Probably a 1 on a scale to 10," Rivers said of his concern level. "Even if he misses a couple of games, it’s not an injury that’s a big deal. I’m not that concerned by it."
Rivers later said he didn't believe Garnett had undergone an MRI because the injury does not appear that severe.
"Again, I don’t think it’s that serious," Rivers said. "It’s just at the point of the year where you’re just not going to play him if he’s not feeling great."
Celtics guard Jason Terry said the team has to be prepared if Garnett isn't on the floor, but is hopeful he'll be able to suit up.
"You can’t think about it,” Terry said of what the team’s mindset would be if Garnett can’t go. “He's not there, so it's not going to help you to think about him.
“But would we love to have him? Yes sir, no question. The best big man, to me, in league history. So when we're missing him we've just got to do it collectively, especially on the defensive end. When you're that small, you have to rebound as a group."
Rivers said the Celtics expect to use a lot of one-big lineups regardless of Garnett's availability, staying small to match up with the Heat.
"With or without Kevin, it’ll be a lot of one big on the floor against them," Rivers said. "With Kevin that means Kevin is the one big, then you can bringing Brandon [Bass] in and out. Without him, that stretches it."
Read on for a few more notes from pre-practice access:
* NO MOTIVATION NEEDED: For all the hype around the Celtics-Heat rivalry and Miami's winning streak (it could reach 22 games on Sunday in Toronto), Rivers downplayed the significance of Monday's game. That said, he still wants his team to get the victory. "What does [a win Monday] get you when you play Atlanta in the first round? It doesn’t do anything that you beat Miami, [another opponent] could care less. If you caught Miami later [in the postseason], I guess for us, knowing that you can beat them, but we already know that. We’ve beaten them earlier in the year. But you still want to beat them, they are the world champs, they are playing great -- you don’t need any extra reason to want to beat them. At least we don’t. They face this every night, though. And it’s what we faced in 2008-09 [as defending champions], every night. That’s what makes their streak so impressive to me, they are getting the best from teams and still winning."
* IS LEBRON THE GREATEST OF ALL TIME?: After gushing about the way LeBron James is playing this season, Rivers was asked if James ranks as the best ever in the pantheon of NBA players. He asked to revisit the conversation down the road. "Well, he’s close. I don’t know who that is. Michael [Jordan] is the best player that I’ve ever seen. I never really got the chance to watch Bill [Russell] or those guys back in their day. My dad died saying Bill Russell, and that’s the way he believed. And right now, I always try to let guys finish their career before we start comparing them to anybody. And I think we should wait for that."
* WHAT ABOUT THAT RAY GUY?: Lost in the hoopla about winning streaks is that old friend Ray Allen makes another trip back to Boston. Rivers admitted he hadn't put much thought into seeing a member of the Big Three yet again. "I haven’t even thought about it, so for me, who knows? I don’t know if our fans will forget that easy. Or should they, I don’t know? There’s a lot of things that go in when you’re playing Miami. Again, they are the world champs, they knocked us out. It’s becoming a fun series."