Garnett, who Rivers said has been completely inactive aside from light shooting since a Dec. 28 loss at Golden State, worked out on an exercise bike and ran Monday, noting he felt "pretty good" after the activity.
Garnett has missed five games since being kicked twice during a road trip out west last month, causing a hyperextension in his surgically repaired right knee.
"I'm pretty sure [Garnett will return] in the next 10 days," Rivers said. "He started working out today for the first time, now it's just a conditioning thing. We want to make sure he gets strong, gets back in shape. He literally hasn't done anything. We figure about 10 days to two weeks, but I figure it's more like 10 days with him. Trying to hold him down for those extra four would be extra difficult."
Rivers joked that it was a "minor miracle" that the team had gotten Garnett to stop all activity since being sidelined.
Garnett appeared briefly in the Celtics' locker room before Monday's game against the Hawks, but did not address the media.
Rivers noted the break has been good for Garnett.
"He's literally not done a thing and that's a minor miracle," Rivers said. "It's been great for him. He feels good."
Rivers stopped short of saying that Garnett's absence was good for the team on the whole.
"I don't know if it's good for us, it's good for him, I guess," said Rivers. "I'm not a big believer in that whole rest stuff, like the new talk in the league over the last 3-4 years. Hell, I don't remember Michael [Jordan] and them sitting out 3-4 weeks so they could be ready for the playoffs, I really don't. I think players like to play, personally. But in this case, because it did get injured, it's the smart thing to do. It was only going to get worse if he played."
Rivers was asked if he had planned to give Garnett days off during the rest of the season, but said he preferred simply limiting his minutes.
"I would have if he needed it, but it's not something I was concerned with," Rivers said. "I do try to limit minutes. But I really think players want to play. When you sit them, it throws them more out of rhythm."
Chris Forsberg covers the Celtics for ESPNBoston.com.