Greg Mattison was always a fan favorite while in Ann Arbor, until he wasn't. Greg was considered the only competent coach on Hoke's staff and many wanted him to be named the interim coach when Coach Hoke was imploding in 2014.
Then Coach Harbaugh is hired and Jimmy keeps Greg on the staff due to his experience and his relationship with the Harbaugh family. Mattison had worked for Jack at Western Michigan from 81-86 and was the DC for John for the Baltimore Ravens from 08-10.
This of course was the second stint for Greg at Michigan.