Kalani has said they want to redshirt him.

http://byucougars.com/m-football/cougars-compete-final-scrimmage-fall-camp-les
Sophomore QB Tanner Mangum
On his role
The coaching staff is not planning on redshirting me. I haven’t heard the specifics of exactly when I would play, but I am not worried about that. It’s just one game at a time right now—it’s a long season, but right now we are just focusing on Arizona and we will just take it from there. There is a lot I can learn from Taysom and Ty. Being there watching the plays and watching the defense and to be able to talk it over with Taysom and Ty benefits everyone. I am definitely not taking a back seat or anything. I am going to stay me and be me and go out there and have fun and do the best I can to help Taysom and to help the team. That’s what we are all about here.

http://www.sltrib.com/home/4279942-155/byu-football-tanner-mangum-stays-positive
"Coaches said Tuesday that Mangum, who turns 23 next month, won't be asked to redshirt in an attempt to "save" a season. He confirmed that, but also said he won't ask for a redshirt late in the season if he hasn't seen any playing time."

“We just know that Tanner is going to play,” he said. “He’s not going to redshirt and we’re not going to save him. He’s going to be a backup quarterback and Taysom will be the starter.”

http://www.standard.net/BYU/2016/08/23/BYU-names-Taysom-Hill-starting-quarterback-2016
"Sitake told the media after practice Mangum will not redshirt and will be the Cougars’ backup quarterback."

Now your turn find me something that says otherwise.