He was on the sidelines long before he became a head coach.
I think he's comfortable there too. I think, watching his press conference, he seems to acknowledge that it's easier to call a game from the booth. However, it's better as a HC to be on the sideline. It'll be interesting to see how it works out. Paterno was in the booth last year for Penn State, and they seemed to have a pretty good season, so I don't see why it couldn't work.