The likelyhood is that if Williams lights it up running the ball then Fiedler is having a good day passing. If they have to concentrate on the run so much, Fiedler is good enough to pick apart the secondary. It's when they key on his receivers that he has trouble. Couch is questionable, so don't even think about him this week. Matthews is a fine choice, I just hope him and Danny-Boy don't rotate.
P.S. The Lions secondary stinks, Fiedler might have a good game.