Morris is sort of an enigma. Based on some reports, Morris is a good fit for this offense as a rusher, in our offensive blocking scheme with the ball in his hands, including being a good short yardage/goal line running back. What he reportedly is NOT is a good pass catcher which is very limiting and is a much bigger drawback if you don't have a back on the roster that can perform that task. If Morris was the backup, and Zeke were to be out for an extended period it definitely wouldn't help his case to serve as the backup. Moreover, he is reported to be a below average pass protector, while McFadden is an above average pass blocker.
I'm not sure that we can assume that McFadden will give way to Morris for a roster spot. It could happen, but I don't see it as a foregone conclusion.