"I'm 36 years old," Bruschi said at a news conference. "Your body doesn't heal as quickly."
He came back from a mild stroke in February 2005 to play eight months later.
"I was retired," he said. "I didn't think it was possible" to play again.
Bruschi's performance declined last year and he was playing with the second unit on defense this summer. He missed much of training camp with an undisclosed injury but played in two of the team's last three exhibition games.
Coach Bill Belichick, usually stoic, choked up as he discussed Bruschi.
"How do I feel about Tedy Bruschi?" Belichick said. "He's a perfect player."
"That's something you'll never hear during your career," he said as he and Belichick laughed. "To have him say that to me is probably the best compliment he could ever give me."