If i may, It's not really realistic though. If the guy is powerful, as you stated in your initial post, then it can't be as simple as demoting him or putting him on the shelf because then he might gather support and start a coup against you. I mean if you have just been appointed Boss, you want to solidify your leadership and have unity within the family. So if your first decision is to get rid of one of the most powerful member of the family because he is not your ally then it will create tensions within the family and your leadership can be exposed. Not to mention, the commission would not look too kindly of such move.

The way i see it, part of being a Boss is being able to manage the different factions within your family. History has showed that the most successfull Bosses were the ones who could coexist with some of their rivals.