I think Doc definitely needs to make some adjustments. I blame the current losing streak mostly on the players, and then on Danny Ainge for doing nothing to fortify the bench. However, Doc certainly isn't getting the most out of his team. They don't appear motivated, which generally falls on the coach's shoulders.
Also, of course, is Doc's substitution pattern. He *needs* to mix the starters in more with the bench. You can't throw a lineup of House/TA/BBD/Powe/Ray out there and expect them to produce. 82games.com has done studies in the past that show that 1) Doc is one of the coaches who uses at least four bench guys at the same time the most, and 2) teams that play 4 or more bench players get outscored consistently during such times. That should be obvious, and yet Doc is constantly putting terrible lineups on the floor to match up with a mix of the other team's starters.
If Doc mixed in the bench with the starters more, I think you'd see the individual games of the players improve. Look at the "player pairs" section of 82games.com. Tony Allen and Leon Powe both play significantly better when they're on the floor with starters. BBD's game even goes up a bit, even though he's been pretty consistently bad no matter who he's been playing with.
People talk about Powe's game decreasing. I think part of that is because he's generally paired next to BBD and three or four other bench players, which doesn't allow him to maximize his game. Last season, Powe only played 264 minutes next to BBD all season. This year, he's already played 336. When the other guy you're playing beside on the front line is shooting sub-40%, it's bound to injure your game, and that's what's happening with Leon.
Doc needs to recognize all of this, and rely on his starters to boost his bench. I know Doc wants to get the "big three" rest, but here's the problem with that logic: it leads to losses. If the season ended right now, to win a championship the team would have to win a home series against Miami, then road series against Orlando, Cleveland, and Los Angeles. That doesn't bode well for the Celtics chances at a title.