Letting Doc Rivers go to the Los Angeles Clippers was the first sign. Getting rid of Paul Pierce and Kevin Garnett clinched it.
The Boston Celtics are rebuilding.
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.
Pierce is the longest-tenured member of the team, its captain, a 10-time All-Star and a likely Hall of Famer. Garnett is also a future Hall of Famer, though only the last six years of his career were in Boston.
That's why Pierce's departure is the one that signals the end of an era in Boston.