Reading the comments from those who seem to know what's what, it appears that Mills was on borrowed time with NP anyway due to indifferent performances and an inflated ego. However, taking the piss out of the manager was always going to get a reaction from NP. Mills gets frozen out as a result and he will either correct the issues he has or needs to move on.
I think it's fair to say that Mills has had some issues since joining us in his personal life. There is also a burden to being such a overly priced player, instantly named captain. This wouldn't make him popular with certain other players in the squad unless he turns out sufficiently good performances to justify this, which he clearly hasn't.
Bottom line is that I think he is a better defender than he's shown us so far. He's also at the crossroads of his career. If he wants to improve and get on, he needs to buckle down, get onside with NP and learn and develop. If he fails to do this, I see his career sliding backwards.
Another point in this that is worthy of a mention. NP has these standards and expectations that make perfect sense. However, there are a lot of especially Premier League footballers with inflated opinions of themselves and if we are to progress to that level, NP needs to find better ways of dealing with them.