I think it is tough to blame Doc. He is doing what he can with the players he has. Rondo can't play this style offense because simply put he has a bad outside shot and defenders cheat off of him. It will be very difficult next year to reincorporate Rondo into this offense because it simply doesn't fit his skill set.
Rondo's effectiveness in our offense came from a very calculated offense which depended on great court vision and incredible timing. He is great at finding guys that can't necessarily create for themselves. The two best examples of this are Ray Allen and Brandon Bass. Another player I might include would be Wilcox. It also didn't hurt to have PP and KG alongside him. In my opinion, Green, Bradley, and Sullinger are the team's future and these players don't need a "true" point guard, they need a guard who can consistently hit 18 footers.
My whole thing with Rondo is that it is incredibly difficult to build around him. I think if the Celtics were to trade him they should move Bass along with him since they compliment each other. Call every GM and say the two are available as a package for the best guard (that can also hit an outside shot) or center.
In terms of what he will be asked to do when he comes back, we will asking Rondo not be be Rondo, and if he is not Rondo then who does he become? He is not a good shooter so there is not really a point in moving without the ball or trying to get open outside. He just adds one more pass needed to find a guy who can shoot. Defenders can sag of him, the court shrinks again and there is one less player who can put the ball in from outside.