I'm thinking you wait until Texas to bring Lanning out. The next two defenses are bad but they'll be games where the offense is in desperation mode after the first quarter. He has to have built up some confidence in how he's played, although sparingly, but I'd hate to see all of that lost while trying to keep them in a game where the other team is scoring 50+.
Why? He is a 20+ year old, 235 lb football player, not some delicate flower with emotional issues. Get him out there and get some experience against a poor TCU defense. Let him sling the ball around the field when we get way down and see if he can get in a rhythm and get on the same page as our good WR group.
We are the only fan base that is so worried about confidence... get him out there and let him play football!