As measured by Defensive Rating, or points allowed per 100 possessions.
According to basketball-reference.com, the Celtics were 12th in Defensive Rating last season. They were at 104.5. Portland was 10th at 103.7. League average was 105.6, The Warriors led the league at 101.4, followed by the Spurs at 102.0. Minnesota was easily the worst at 112.2.
Their defensive rating in the Isaiah Thomas era was 103.1. Their defensive rating with Rondo was 106.3. Their defensive rating in-between those two periods was 106.2.
The Celtics added defensive sieve David Lee, but they've also added potential defensive anchor Amir Johnson. Was the late-season improvement on defense a fluke or is it sustainable with increased continuity? If the Celtics can be a top-ten defensive team, should they be considered likely to make the playoffs?