Manziel, Jameis Winston, Mariota, Vince, Colt, Andrew Luck all redshirted - as did the vast majority of elite college QBs. Yes there are exceptions (I believe RGIII played as a true frosh) but usually the college coaches only play them due to desperation at the position.
The worst thing that can happen to a QB is busted confidence. Just look at David Ash - a guy comes out of high school brimming with confidence and then you throw him in unprepared and he gets pounded b/c it is the hardest position to play mentally and physically - then the team and Ash view himself in a lesser light due to having some poor games and that at ude is hard to get rid of. I always wonder how Ash would have panned out had he gotten a full year to develop.
You only play a guy out of desperation and that's why I'm afraid Mack does play him this year - which will be a mistake long term. Especially considering there is no scenario where it would be worth the trade to play him due to how the year has already panned out.