Mac (or whoever is the #2QB), absolutely has to get some meaningful playing time, running the whole offense the way it's designed to be run -- deep balls, short releases, RPOs....everything. It can't be garbage time, handing off against a demoralized defense just to chew clock.
We don't know whether Tua will miss time due to injury.
We do know:
1. The chances of Tua being on campus for the 2020 season are roughly zero.
2. Hurts is gone, so we no longer have a proven backup.
3. Tua's been hurt a lot in a relatively short period of time, which would indicate that there's a good chance we'll need a capable backup at some point during the season.
Even if Tua makes it through the season unscathed (and his history would say that's unlikely), we don't need to have a QB lining up for his first action against USCw on 9/5/2020.