Howard Beck, nytimes.com
Howard Beck, the N.B.A. reporter for The Times, answers questions from readers.
The Celtics are one of the NBA's few underachieving teams without a built-in excuse. Kevin Garnett, Paul Pierce and Jason Terry, all 35 or older, have yet to miss a game. The same goes for everyone in their nine-man rotation but Rajon Rondo and Avery Br…