I think Glennno will start for a few weeks and if he is not doing well, then Mitchell will replace him. Not just in the season, but the future would be now.
It would shock me if he started in day 1. As IMHO, if Glennon flops, Mitch could be. But for this year, he will learn under Mike G. Glennon could start in week 1. But in grooming him he could be in the top 5. And hopefully grooming him rightfully.