Just in case you need some numbers to back up what your eyes tell you - the Celtics are playing some very good defense.
Defense has been key to the Celtics’ resurgence - The Boston Globe
The Celtics have not allowed a team to score 100 or more points since the second game at Miami, when the Heat scored 115. Although their defense has been a strength all season, the intensity and precision has increased the past nine games. In that stretch, behind the rejuvenated Kevin Garnett and the ball pressure of Avery Bradley, the Celtics have allowed opponents just 81.2 points per game on 38.8 percent shooting. Only the Cavaliers and Wizards have managed to shoot more than 45 percent and the Magic and Raptors combined for 120 points. Boston is second in the NBA in points allowed (86.7, behind the 76ers), which has been a savior because the Celtics are a stunning 25th in scoring.
I guess defense wins, but it would help to get a little better at offense too. Just a thought.