Two big things to keep in mind:
1) Most of his minutes today he was playing off the ball either with Rondo or with Ray handling the ball the majority of the time when Nate was technically the "point guard." Doc needs to keep this in mind, either pairing him with Rondo or letting someone like Ray, Paul, or Daniels handle the ball when Nate's in the game.
2) Nate plays so few minutes now that it's easy to understand why it's hard to snap out of a slump. He literally plays in most games what Paul, Ray, or Rondo plays in a quarter.
Like other players on this team who have had to adjust to new roles (Ray's initial struggles here come to mind), I think Nate is going through some growing pains, learning how to be effective in the fewest minutes he's ever played in his career.