Without a close second, over the first month of the season the Celtics biggest flaw was it's three point shooting. We went over it, extensively. But to recap, coming into the month roughly 25% of the Celtics field goal attempts were 3 point shots, and they were hitting only 30% of them with no substantial difference between contested shots and shots that are wide open. The last parts important, because it emphasises that the Celtics weren't just missing shots - they were missing the right ones.
Luckily, things are starting to correct themselves. Per Jay King:
Over their last nine games, the Celtics are shooting 37 percent from behind the arc.