Bill O'Brien is staying at Penn State after a brief flirtation with the NFL, but one of his top assistants and closest friends is not.

Defensive coordinator Ted Roof resigned Wednesday after spending just one season at Penn State, and took the same position with Georgia Tech, his alma mater. O'Brien acted quickly in promoting Nittany Lions secondary coach John Butler to defensive coordinator.

Few are surprised to see Roof on the move again, as Georgia Tech becomes his fifth coaching stop since the 2008 season (Minnesota, Auburn, Central Florida, Penn State). He has strong ties to the state and to the school, where he served as linebackers coach in 1998 and defensive coordinator from 1999-2001. But he's also extremely tight with O'Brien, who he worked with at his previous stop at Georgia Tech. O'Brien later joined Roof at Duke when Roof served as Blue Devils' head coach.

"It's very important to keep your staff intact, but you have to be careful there, too, because I think it's important for these guys to want to move up the ladder," O'Brien said Monday at a news conference. "So you've got some fantastic coaches on our staff that are going to have chances probably, maybe not this year but maybe in other years, of being coordinators or even head coaches. You never want to hold those guys back from being able to do that, and I never will. But obviously from a continuity standpoint, you'd love to keep them intact and do the best you can with that."

Regarding assistant coach salaries, O'Brien said, "These guys are paid well. I think that they can always be paid better, and I think there's things that we've talked about from Day 1 here, [athletic director Dave Joyner] and I, about ways that we can do that, whether it's a bowl bonus or what bowl would we have gone to if we win a certain amount of games, things like that. So those are conversations that take place, and like I said, in the inner circle, and we'll do the best we can to continue to try to make it attractive to coach here."

It's hard to blame Roof for heading home, although Penn State had a better season than Georgia Tech, and the PSU job has more national prestige. You have to wonder how concerned Roof is about Penn State's future with the NCAA sanctions, despite the team's success this season. Although Penn State held onto O'Brien for at least another year, it's critical for the school to retain assistants and maintain staff continuity.

Roof didn't immediately return a message from ESPN.com. Penn State's defense finished 16th nationally in points allowed, 23rd in rushing and 29th in total yards allowed in Roof's sole season as coordinator.

O'Brien went with Butler at defensive coordinator rather than veteran Penn State assistants Larry Johnson (defensive line) or Ron Vanderlinden (linebackers), the only holdovers from Joe Paterno's staff. It's unknown whether O'Brien offered the job to either Johnson or Vanderlinden, who might be content in their current roles.

Before joining O'Brien at Penn State, Butler served as special teams coordinator and helped coach linebackers at South Carolina. He held a similar post with Minnesota from 2007-2010. I got to know Butler a bit at Minnesota, and he struck me as an extremely bright coach. This is certainly a big step for him, although Penn State's structure of success on defense is already in place.