It's very tough for this team to win games when Pierce is playing as badly as he has been recently.
It's not just a normal bad game for a star talent. Pierce has been fluctuating wildly from All-Star caliber or near All-Star caliber play to playing like a lousy backup SF.
That is a huge decrease in overall contribution. It's very hard for any team to lose that type of production from one of it's star players and still play at a high level. Boston, clearly cannot do so.
The Celtics offense is too reliant on Pierce to survive his loss. Heck, the three main guys -- Pierce + Rondo + Garnett -- whenever any of those guys are out of it, it is very tough for Boston's offense to survive.
And, I think that is partly down to Doc Rivers for failing to open up the offense more this year and get more out of an offensively talented supporting cast.
Rondo is the guy who creates good to high percentage shots for everyone. When he isn't around, the Celtics offense devolves into a series of ineffective one-on-one plays.
When Pierce is missing in action, Boston loses their best shot-creator and nobody steps up into his place. Jeff Green isn't talented enough. Rondo isn't willing enough (to take that added scoring burden).
When Garnett isn't there, then Boston gets almost no offense from their big men corps. They need his high post play (jump-shooting and passing) to facilitate the team's offense and occasional low post play to give some variation to it's sets.