You can't blame it all on Doc.
The Celtics don't have a go to guy when things get tough. Don't argue about it; they really don't. Pierce isn't that guy anymore. Neither is Ray. Neither is KG. Rondo isn't there yet. All of those 4 guys are still very good players, but they aren't guys you can go to when things get rough and depend on them to pull your rear end out of the fire.
You need a go to guy if you want to win big games, and ultimately, a championship. No team in the past 20-30 years (as far as I'm aware) has won a championship without a real go to guy. Even Detroit had Mr. Big Shot.
The lack of a go to guy is why this team sputters late in games when the opposing defense clamps down and they can't get the same easy looks they got before. They don't have a player who can demand the ball and make things happen single-handedly no matter what the opposing defense does.
You look at all the best teams in the league, they have that guy. Cleveland has LeBron. The Lakers have Kobe. Nuggets have Melo. Atlanta has Joe Johnson. Mavericks have Dirk (and Terry, too). Even the Jazz have D-Will and the Magic have Rashard, Vince, and Nelson; even Dwight has become more of a clutch player recently.
There are nights when one of the Big 4 will look phenomenal. They might even hit a game winner or come up big in the 4th. But none of them can do it consistently. None of them can really take over a game anymore - not against a really good team. Unless that somehow changes, I am very skeptical about the Celtic's chances of playing in June this year.