Sure, then you buy him out. I think all the new assistants will need buyouts as I understand they generally got 2 year contracts. It’s really not a big deal.
A new head coach who ‘hates’ Mickey or has a terrible personality conflict with him is raising some red flags. That doesn’t sound like a very diplomatic guy if he can’t work with a variety of personality types. Yes, the head coach ought to be able to hire the staff he wants, but it’s not unusual for an AD to encourage him to keep a coach or two, a coordinator. It was part of the deal for Notre Dame’s new coach, he had to keep his coordinator.