Belichick has assured us that Harry has the mental stuff down, and that the issue is that he's not yet executing at the level needed. However, despite BB's assurances, we've seen that Harry's been a problem in games when it comes to the mental angle as well as the physical.
So, if we are to believe BB, we know the problems. Harry is not physically executing plays properly, and he's also not translating his understanding of the game from practices to game conditions. Or, as some of us keep having to repeat:
Harry's not ready.
If this team hadn't screwed the pooch on AB/Gordon/Thomas, Harry might not even be active right now.