When you defend, as the Celtics do in spades, you are never really out of a game—even when you have to close with Al Horford and four guys who were in elementary school when he came in the NBA.
And don't overlook the Baynes/Theis combo. The Celtics haven't had a really physical center since Kendrick Perkins left town. Now they have two. Baynes is more skilled than Perkins. Theis is more athletic. But both are tough and want to find contact. That goes a long way, especially in the playoffs where the more physical team often wins.