If it was unsafe to play, Thorson would not have played at all. The strategy for Thorson in the Purdue game was clearly about maximizing Thorson’s effectiveness over an entire 12+ game season. To do that, Thorson needs to play as much as possible, but not so much that he cannot practice effectively the next Monday or Tuesday (due to swelling and/or excessive soreness) or play at a high level the next week.
If Thorson woke up last Friday and Saturday mornings feeling pretty good in the knee, he ought to play more (if not all) snaps vs Duke. If he had excess swelling and soreness, he ought to stay on a pitch count vs Duke.
The best case is that Thorson felt so great the mornings after the Purdue game, that he (and the coaching and medical staff) thinks he can play all competitive snaps vs Duke. Like any football player, Thorson will get better and more confident by actually playing the game (as much as possible) and making plays. I’d rather Thorson play a full game (while the game is still undecided) before Michigan comes to town. Resting him just to “keep him safe” is a terrible idea. Only rest him if it’s necessary for him to recover for the next game and the next week’s game prep.
I don’t think that trying to make it through a full game for the first time versus a quality Michigan defense is the ideal first test. I hope he can get more snaps vs Duke and all (competitive) snaps vs. Akron.