They are letting in more points, suggesting that defense is the central problem. The major lineup change is that Kendrick Perkins is playing less or not at all recently, and is reportedly not so effective when he is in. He has an injured left shoulder, perhaps worse than the Celtics are willing to admit.
Celts fans were so pleased to have a legitimate center at all last year, that we overlooked the fact that we have no decent backup center. We have had an abundance of power forwards who could also play center, but no true center. This has gone underappreciated with Kevin Garnett on the floor, because he can soak up some of the slack when Perk is playing a full-strength 30 minutes a game. But with Perkins out or only at 80%, it becomes clear that Glenn Davis, Leon Powe, and Brian Scalabrine are all power forwards - decent backups who can play a little center when pressed. Further disguising this lack is that fact that a fair number of other teams do not rely on a true center for more than 20 minutes/game, so the matchup problems are not immediately apparent.
Because Perkins is probably the least talented of the starting five, it seems counterintuitive to think of him as the most indispensible on the roster. The other four are not easily replaceable for long stretches, but all have people who can step in adequately for a game or two.
The defense has not adjusted to the lack of a center. As witness tonight: Toronto's Center Bargnani, who usually averages 12 ppg in regulation has twice that many tonight, and more rebounds as well.