The Celtics have agreed to the terms of a deal that would send the two remaining members of the Big Three that won the 2008 NBA title to the Brooklyn Nets for a package of draft picks and players. Garnett is a future Hall of Famer, but it's Pierce's departure that signals the end of an era for the league's most-decorated franchise.
''It's sad to see everybody leave Boston. You just want them to go someplace where they have a chance to win, and they have,'' Rivers said at Clippers draft headquarters late Thursday night. ''It's a great trade for Boston, too; not now, later. Danny wanted to rebuild, and that's what he's doing.''
The longest-tenured member of the Celtics, Pierce is the team's captain, a 10-time All-Star and a likely Hall of Famer. He is the second-leading scorer in the history of the NBA's most-decorated franchise, and also is in the team's top seven in rebounds, assists, steals, games and minutes played.
Garnett is also a future Hall of Famer, though only the last six years of his career were in Boston. It's Pierce, who slipped to 10th in the 1998 draft and has been a Celtic ever since, who had a chance to spend his entire career with the franchise and add his name to a list that includes Hall of Famers Larry Bird, Bill Russell and John Havlicek.
(But not Bob Cousy, Robert Parish or even Red Auerbach.)
''(It's) sort of sad. You hate to see it,'' said Chicago Bulls coach Tom Thibodeau, who was an assistant on the Celtics 2008 championship team. ''But that's the NBA. It's constant change, and you have to be ready to adapt. I think what Paul Pierce did for that franchise and Kevin - I think's it's good for them. They have an opportunity to continue on.
''Good for the Celtics, where they can start their rebuilding.''
Pierce was drafted in the Rick Pitino era when the Celtics, already in the midst of the longest championship drought in franchise history, were a year removed from the second-worst record in the NBA (a mark that was not good enough to land them the top prize in the draft, Tim Duncan).
He helped the team reach the Eastern Conference finals in 2002. But, convinced that they were not likely to go farther, Ainge was brought in the next year to tear things apart again.
Boston plummeted back into the NBA lottery and again bad luck prevented them from landing a franchise player like Kevin Durant. Instead, Ainge swung deals for Garnett and Allen that earned the Celtics their NBA-record 17th championship in the very first year.
But Garnett was injured the next year, and Kendrick Perkins went down in Game 6 of the finals in 2010, when the Celtics lost to the Los Angeles Lakers in seven games. Then came two eliminations by Miami and, after five straight division titles, a third-place finish and first-round loss to the New York Knicks.
Ainge was convinced that the time had come to start over.
Rivers' exit was negotiated with the Clippers, landing Boston a first-round draft choice in 2015 and freeing the Celtics from the $21 million remaining on his contract. Garnett and Pierce will go to Brooklyn as soon as the deal can be finalized on July 10.
Tim Hardaway, a scout for the Miami Heat, said: ''We don't have to worry about Boston no more.''
Not next year, at least.
But the Celtics will have two first-round picks in four of the next five drafts, including the loaded one expected for next year.
''The Celtics wanted draft picks and they wanted to drop contracts. I was one of those contracts,'' Rivers said from California on Friday in an interview with CBS radio in Boston. ''It's a hard thing to do, but I think it's the right thing to do.''