You play your best players. That's how it works. If Tua doesn't get injured there are you going to say "should have only played the backup QB"? Should they have sat him until he was 100% healthy and just bet on the come that the back up would get them to the playoff? That isn't what they did last year. He stated he was 80% healthy going into the playoff.
Bama is currently a 1 loss team that isn't going to the conference champ game. They need to win convincingly to get a championship shot. Aside from that, no team has ever been up 28 and pulled players in the first half. Not when college football teams consistently score at the rate they do these days. MSST has scored 14 or more in a quarter in all but 3 of their games this year, leading up to the Bama game. They can score some points even though they suck shit through a straw.
You've got a bad take, but that's ok. You play to win the game, maybe Saban knows he isn't going to win shit with the back up. He's got a lot more wins than you do, I'm going to go ahead and trust his judgement being that the Bo Jackson injury has happened about 5 times in the history of football.