Look what this system has helped produce: five head coaches, including the Atlanta Falcons' Mike Smith and the 49ers' Mike Singletary, and Ryan is in line to become the sixth.
For a lot of Ravens, such as defensive tackle Haloti Ngata, linebackers Terrell Suggs, Bart Scott and Jarret Johnson, cornerback Chris McAlister and safety Ed Reed, this is the only system they've played during their NFL careers.
And then there is the Ray Lewis factor. This is a scheme built around Lewis; he can run untouched from sideline to sideline to make tackles. As Nolan would often say, "This is Ray's world."
Nolan would be an ideal replacement, but there are reports that he will become the Denver Broncos' defensive coordinator. Marvin Lewis would also be a great candidate, but he can't get Bengals owner Mike Brown to fire him.
To bring in just about any other outsider would be a tough sell, especially if he wanted to install his own system. With a defense that has been as dominating as this one the past nine years, you want someone who not only knows the system, but who also knows the strengths, weaknesses and souls of the players.
One of the keys to the Ravens' success in 2008 has been the smooth transition to Harbaugh. It's a luxury to retain a coach such as Ryan and several of the assistants from the former staff.
Instead of having to butt heads with the dominating personalities of Lewis, Reed, Suggs and Scott, Harbaugh had Ryan a buffer. Harbaugh could send Ryan to solve any problem first, and if that didn't work, then Harbaugh could step in.
But those days are close to being over. In Ryan's four seasons as defensive coordinator, the Ravens have not finished below No. 6 in the league in total defense. His players play hard for him because he has a stern, but not overwhelming, personality, and he likes to have as much fun as they did.
That's going to be tough to replace.
Who's next? We don't know.
Listen to Mike Preston on Mondays from 4 p.m. to 6 p.m. on Fox Sports (1370 AM).
