One of the biggest issues with the Celtics' offense is what I mentioned before -- the Celtics, for quite some time now, have had to settle for outside jumpshots late in games because they have little to no inside game. That goes hand in hand with the lack of rebounds.
So even in games like today where the offense is great for a half, the game tightens up and they get held to 25-30 points in a single half, which is abysmal. That sort of streakiness happens when you rely on jumpshots almost exclusively for your offense.
The fact that the team gives away a lot of possessions not just by allowing offensive rebounds and failing to get any themselves, but also by turning the ball over a lot, just compounds their problems.