I don't think this is a poorly coached team or a poorly managed team at all. What I believe is that we're seeing the combination of not having any above average offensive players and unrealistic expectations.
Yes, Paul Pierce is a very good offensive player - in stretches. At 35 years old, he just can't do it consistently anymore. And honestly its not fair to expect him to. Jeff Green is talented guy, but his offensive game lacks versatility (for starters: he can't go left, he has no pull up jumper) necessary to be a #1 option. Even when Rondo was healthy, he's more suited as a complementary guy who can be dangerous - he's more of an opportunistic scorer than a guy who can give you 15-20 pts consistently. What we have is a team full of 2nd, 3rd, and even 4th offensive options. We were always going to be an offense by committee and therefore offense was going to be our Achilles heel.