This is so much less fun to do after a loss, but such is life. Let’s get granular (and humorous):
Early turnovers and bad execution put Boston in an inescapable hole.
Looking at the box score, you can see the Celtics racked up only one turnover more than the Nets (19 as opposed to 18). Brooklyn is not particularly adept at forcing turnovers (or preventing them, as Basketball Reference shows), but they managed to do so early and often in this contest.
This contributed greatly to Boston’s inability to ever punch back and maintain a revitalized status, despite having very good runs near the end of each half.