The best possible scenario I can imagine, is for a graduate transfer to come in and take those mop-up snaps. Mac doesn't seem ready, and Tua while showing flashes of brilliance might not be either. A graduate transfer would give a veteran presence at the position and allow for mop-up duty without burning a redshirt.
Having said that, that requires three things basically. A: It couldn't interfere with any promises Saban might have made, B: It requires a free scholarship, and C: It requires a graduate transfer that could fill that role. But, that's the ideal scenario in my mind. If something were to happen to Hurts, for instance causing him to miss multiple games, then I think taking the redshirt off Tua could become the best choice. But, using up a year of his eligibility in mop-up duty? I'd just hate to see that and it seems to me it would just up the chances that Tua transfers (basically putting him in a win the starting job or transfer position).
As of right now though, given the options, Tua is the #2 and he won't be able to redshirt.