You keep bringing in young guys to backfill your roster. When you get them here in spring, you evaluate them on academics, community service, workouts, and spring practice. Your first cuts are the ones in their 3rd year and up if they're getting passed by younger guys. Then you look at the "OOU" mentality and cut the ones who are not passing that test. You tell them where they're at in the roster and tell them that you just don't see them playing at Nebraska; then pass them their options.
From there, you keep doing that until you make that 85 man scholarship. Prescott and Wilhite must have been very strong evaluations to bring in and force a few more cuts if necessary.