Tomlin has already done everything he can. He cannot have a bad day. If the Steelers have a bad day because they are not prepared that would be bad for Tomlin. I don't think that is the case, though.
If Ben screws up, Batch will not come in. Unless you are prepared to start Batch the rest of the season (and that would be stupid) you don't replace your starting QB just because he's not playing well. The only time you will see Batch come in is if Ben is injured.
With that said, I hope they get up big in the first quarter because I have to coach 11 and 12 year olds at 2:30. Our biggest problem will be trying to get 46 kids their required 4 plays in a game with 8 minute quarters.