I think Jimmy values NFL time and he values the PSU system/principles.
Donovan has basically nothing on his resume that says “this guy did this and it was really good,” but he’s been around some things.
He was with James Franklin in his first year at Vandy. He was with Franklin at his first year in PSU. He knows how to deal with a whole batch of brand new kids and install stuff and been with a first year coach.
He’s been in the NFL, which is probably why Jimmy had such a hard on for Kellen Moore.
He has experience with multiple offensive position groups, he was even a DB in college.
Maybe Jimmy couldn’t get anyone better (first year HC) and he trusts this guy to run things and help him out.