My first thoughts go to the Celtics defense, or lack of it. With basically the same core group as they had last season that had the #1 defense in the league, the Celtics are now 24th with a defensive rating of 117.0. It's not the absence of Robert Williams because they kept their defense strong and at the top while he was out after the meniscus tear last season.
If anything, the addition of Malcolm Brogdon, who is a good defender, should have added to last season's stellar defense. But the Celtics' defense has been porous at best this season.