I don't think that's the main problem with our starters. Surely, the offense is not clicking as it should, but the main problem with our starters so far has been their collective defense in my opinion. Pretty much all teams have scored on them in bunches in the 1st quarter. But as you say, the offense has not been good for them.
Another problem has been our turnovers... so far this season it has been travelling and butter fingers central.
Another is the passiveness from our PG. I can't say it enough, but Rondo has to start looking to penetrate more. Even if he's not looking to score, he should at least threaten to do so. He crosses half court, and doesn't look at the basket even once. Not putting the blame on him for our struggles, since we are not shooting well either, but he's the PG and it starts with him. He should be constantly at the meat of the defense causing trouble... forcing the defense to close out on him so that the ball movement improves.
As you say, our 2nd unit has been getting it done, and it has started with their defense.
But, yeah our offense is a huge concern... but so far, in my opinion, it has been the collective defense of our starters (especially in the first quarter) that I've been mainly unhappy with.
We might be 27th in the league in points per game, but our FG% at 44% is not that bad.
We're awful in free-throws too. That has played a huge role in our offensive struggles.