Count me in the limited crowd that believes McFadden will be terrific in this offense. That being said, I think it is imperitive the Cowboys trade for AP. Rationale:
Romo is one awkward movement away from ending his career. Not even a hit (remember the Washington game 2 years ago?)
Regardless of the back-up QB, that guy won't be Romo. They will have to rely heavily on the running game to have a consistent chance to win
For these simple reasons, adding a bona fide workhorse RB to an already good stable will:
Reduce the reliance on the passing
Plays to the strength of that tremendous OL
Shortens games, taking a load off the defense
In case of injury to Romo, still gives the Cowboys a fighting chance to do some postseason damage
To me, giving up a 1st or 2nd round pick is a no-brainer. Restructure AP's contract making it more cap friendly, and sit back and enjoy the most prolific offense in football.
