But Bird believes that the aging stars will be more than productive with the Brooklyn Nets.
"I think they've both been pretty healthy throughout their careers," Bird said Monday while speaking to reporters following the Indiana Pacers' Orlando Summer League game. "They haven't had the major, major injuries that I had.
"I think they got plenty left. The way they will be coached and the bench that they have -- yeah -- they'll pick their spots. But when the time comes, they'll be there."
Pierce and Garnett were dealt to Brooklyn in a blockbuster trade last month, signalling Boston's intention to rebuild with younger players.
Bird, one of the greatest players in Celtics history, currently serves as the Pacers' president of basketball operations. The Hall of Famer acknowledged that it will be difficult to see Pierce, the second-leading scorer in Celtics franchise history, finish his career somewhere other than Boston.
"I would have loved to see it," Bird said. "I got a lot of respect for Paul and what he has accomplished. But who says he's not going to play another four years? It's a tough situation."
Pierce, 35, averaged 18.6 points per game this past season and trails only John Havlicek on the Celtics' all-time scoring list.
"[Pierce] is one of the better ones to ever come through there," Bird said. "He really is."
But Bird also claimed that he understood why Celtics president of basketball operations Danny Ainge traded Pierce and Garnett.
"Believe me, Danny Ainge knows what he's doing," Bird said. "He won a championship. I never doubt Danny Ainge. ... Danny does a good job. You know, you do get older. You can't beat Father Time, and I'm sure things will work out."