Doc is simply not a top level coach like they thought he would be. People say one of his strengths are motivation, but his celtics squad had some of the best motivators like Pierce and KG. And here we are seeing that Doc's motivation aint helping them boys, last years choke was horrible. The Rivers rotations are questionable, sometimes he plays the rotation without cp3 and griffin for a good chunk of minutes where they look helpless out there. Rivers coached teams lost two series where they was leading 3-1 and if I am right three series where they was leading 3-2, it shows that Doc can't do his homework on making right adjustments to close out the deal. He is just a really overrated coach.
Of course there are plenty of much worse coaches out there but just saying if you give the rights to the coach to be your gm also you better make sure he is capable of doing that, at this point doc rivers is just living off his celtics fame.