During an appearance on The Vertical podcast, Rivers said he believes Pierce hasn't made up his mind about his future.
"If Paul decides to retire, then we're going to make sure that Boston picks him up for one day and he retires a Celtic, because that's what he should retire as," Rivers said during the podcast released Thursday. "So we have all that in place. We just don't know what he's going to do."
Added Rivers: "I think Paul's going back and forth on it, and I want to give him room. I think he deserves it. From a team perspective, you would love that he made his decision eight weeks ago. I think there are certain guys that you just have to give time to, and Paul's one of them."
Pierce, 38, played for Boston for 15 seasons before spending one season apiece with the Brooklyn Nets, Washington Wizards and Clippers. He was the NBA Finals MVP in 2008 and is the second-leading scorer in Celtics history, behind John Havlicek.
During an appearance on ESPN's The Jump last month, Pierce admitted he's "50/50" on whether to continue playing.
Kobe Bryant and Tim Duncan already retired this offseason, and Pierce and Kevin Garnett remain undecided on their futures. On The Vertical podcast, Rivers said, "It may go down as the greatest retirement summer in NBA history."
Addressing unsubstantiated rumors that the Celtics and Clippers have talked about a deal involving Blake Griffin, Rivers smothered the suggestion.
"[Celtics president of basketball operations] Danny [Ainge] and I have talked twice this summer. One [call] was about the British Open, and one was about another golf tournament. That's about it," Rivers, who handles personnel decisions for the Clippers, told The Vertical podcast.
Added Rivers: "It's funny, you don't want to go out and send out a press release every time there's a rumor about Blake. We are hoping that Blake ends his career playing for the Clippers. Period. So when teams call, we say we have no interest. First of all, no team's calling right now because teams know we don't have any interest.
"It just tells you the different times. Things have changed. Everyone believes that they're media now. ... There's so many good, credible guys. And then there's some of the guys who are bloggers who have nothing to do with the sport.
"I helped my young son trace where this started -- the Blake rumor, the Boston rumor -- let's go there. ... My son traced it to, I think, it was a Boston radio talk show. And the guy didn't say that we had been talking, he said Blake would be one of the guys that the Celtics should go after. That started the next step, to the next step, to the next thing you know it blew up. And that stuff is tough."