This isn't a confirmation from the team, but Bryan Broaddus is a knowledgeable Cowboys source and his guess is that Darren McFadden, not Alfred Morris, would replace Ezekiel Elliott if the Cowboys' starting RB were to miss time due to a suspension.
McFadden hasn't suited up during the regular season. He was last seen in the preseason rushing 20 times for 102 yards for a yards-per-carry average of 5.1. McFadden is a more versatile back than Morris, and could provide a bigger spark to the offense over the former Washington back, but it's all a guessing game at this point. We currently have Morris ranked ahead of McFadden in our rest-of-season rankings.