Tatum is never going to be that natural transitional player like Bird, Magic, Mike and LeBron. Those players don't grow of tree's.
IMO he actually did try to become that all-around facilitator at the beginning of the season, or at least he gave it lip-service. He found that it was much more difficult than he anticipated and it effected his scoring far too much. At this point he's more just kinda thinking about it, which is an improvement for a 23 year old.
Have no worries, he'll be going back to being "JT" the face of the Boston Celtics, he'll sell some chips and dip, remain a member of that elite NBA All-star club, hug his fellow opposing all-stars at the beginning and then again at the end of every game... Win, Lose, or Draw. Perhaps even take them to dinner after the game. He is quite affable.
It doesn't mean you can't win with him, he needs to be surrounded with players that can keep him grounded and who can do the things he can't.
Think of him as Paul Pierce without KG, Rondo and Ray Allen.