I think it will be hard for Larry Johnson to get much of the credit, unless the D-line just absolutely dominates every opponent this coming season.
Luke Fickell, being the Defensive Coordinator in title, naturally will be given most of the credit by the media.
However, my opinion is that Cris Ash will be the most responsible. To improve a unit 26 spots overall is a pretty major task. While, in retrospect, it is only a difference of some 27 yards/game to move into the top 20, it will be light years away from where his inherited-unit was at last year.