Greg Mattison spent 13 years on Michigan's football staff.
On Wednesday, he talked publicly for the first time as a member of Ohio State's football staff.
“Coach Harbaugh was great, okay. He understood I wanted to coordinate and it was an opportunity to be a coordinator, and that was what I wanted to do. Not much you’re going to do about that," Mattison said.
Hired as the Buckeyes co-defensive coordinator, Mattison called Harbaugh a great coach, and shed light on his relationship with Ryan Day.
Mattison said he met Day years ago and knew "right away" he was destined for big things.
“As far as the rivalry, as far that -- it’s always hard. When you leave people you’ve been with for 13 years, when you leave people at any job, one thing about college football, when you leave one program and go to another... that’s one of the hardest things there is," he said.
Mattison was Brady Hoke's defensive coordinator. Jim Harbaugh kept him on the staff as defensive line coach.
"The hardest thing, probably, was calling the players," Mattison said.