Fason and Morency are the next best runners in the draft after the top three. Arrington is a straight line runner that is highly overrated, but there are a lot of you that overrate a lot of prospects.
After Fason and Morency, I would look to Ryan Moats. He's got a lot more lateral agility than Arrington and has a better build. Ernest Shelton has an arthritic condition in his neck.
Simply put, if Miami passes on a runner in the first round and don't get either Fason or Morency in the 2nd, then they won't have a starting running back next year. Those are the only runners I see with a chance to come in and start as rookies, and the drop off from Brown/Benson/Williams to Fason/Morency is VERY LARGE.