As good as it was for Michigan football to hire Kyle Whittingham as its next head coach, that hire will only be as good as the hires he's able to make to his staff.
In order to keep Bryce Underwood, Whittingham will need a solid offensive coordinator, one with a proven ability to work with quarterbacks. That would help retain some other players on the roster, too.
There is always a fine line between bringing in the best coaches available, while trying to maintain some of the current staff for roster continuity or recruiting.