This is a stretch, but here goes...
Maybe the injury is worse than Jauron reported and Crowell would miss a larger part of the season. Let's say Crowell is healthy enough to play near the middle of the season. The Bills can still actvate him then if he passes through waivers, right? So if the Bills do that, another team would have to be willing to roster Crowell (and cut another player to make room for him) only to have him for a short time before he becomes a free agent. How productive could he be learning a new system mid-year? The Bills might not be all that high on his play and may feel they will lose him anyway at the the end of the year. Maybe they'll take a wait and see approach on his recovery and then roll the dice that he clears waivers IF they want to activate him. If they lose him then, no big deal. Maybe they feel Ellison can play well enough to take this risk and maybe they'll try to make a move to add a LB to the roster now.