the guy that wins the back up job to be the backup. It may be him...it may be Hoge...it may be McCoy Hill...heck it may be Detmer. The only guy I would guess they "expect" to be in the two deep is Taysom. After that, they'll analyze things in fall camp.
I have no doubt there are contingency plans in place based on who steps up. If Mangum is good enough, he likely gets enough reps to be marginally seasoned the next year. If not, then they may plug someone else in as a stop gap to preserve the redshirt year. I would have liked to have Hosick simply because he's a guy who has played recently...but...he saw the writing on the wall and opted out. I don't blame him. It's likely Hill this year and Mangum the next.
But who ends up the number two is definitely not set in stone. And it doesn't need to be...yet.