What I remember, there was no specific statements that Belichick took over the playcalling or the gameplanning.
However :
- The defensive players were questioning the conservative playcalling, that is for sure as we all saw the quotes. Soon after, things changed.
- Mangini lost his red shirt for several games. That is also for sure as we saw it with our own eyes. Who was calling the plays then, who knows. Maybe Mangini but, if so, why was he not wearing the red shirt ?
- Speculation from one of the writers with "sources" (which makes it a little more uncertain) was that Mangini was protecting "his" secondary too much once Harrison was out. Also, equally damning, maybe more so, the "sources" said that Mangini had trouble going from position coach to DC and didn't take and get control of the defensive meetings.
It was all fairly well documented and adds up to some definite issues. But I'm not subscribing to those papers to prove it