If Bohanan was going to be the starter, he's not transferring here to compete for the job. He lost his job at USF, doesn't matter that it was due to injury, he has been replaced with someone USF thinks is better.
If Hillstead had been what Utah State had hoped for, they wouldn't have brought in a transfer to be the starter. Hillstead could develop into a good QB but he's unlikely to step in Day 1 here and be a Big 12 quality starting QB. If he was that already, he'd likely still be at Utah State as the starter.