With Rex, Tebow has a better chance of starting at RB (especially with all of the injuries to that position). Rex never wanted Tebow for the QB position, that much is clear. Like it or not, Sanchez has to get hurt or Ryan gets torched at the end of the season and Woody Johnson hires a coach (if any) that would play Tebow as a true QB. It is obvious that Mark can't play himself out of the position, or that Tebow will have a chance to earn it.
