Guys come on. You can't just install entire new formations and plays in the middle of an NFL season. These guys aren't brain surgeons. There are only a couple of real practices in the week, and in those there is barely enough time to install new personnel packages into the existing scheme and a few play tweaks perhaps. In training camp, we built the offense for Romo. It was designed to protect him and establish the run while having Dez draw doubles.
Weeden steps in and can't run that offense. Cassel steps in and probably knows part of the playbook, but not all of them second nature. Now you want to run a spread with a guy who barely grasps the base offense. Nope.
If in a NASCAR race your driver breaks his arm mid-race, you can't take the time to retool the car, adjust the seat/steering/pedals for his benefit. Right now our replacement driver can't see over the dashboard even on the phone book he's sitting on, and is reaching to push the pedals with the tips of his toes. Now you want to switch the gauges he's looking at to the metric system.