Do you mean Ridley? Shanahan will run Morris into the ground, just as he has done with every one of his runningbacks, ever.... It may not happen this year, which I understand is the entire point of fantasy, but don't be surprised if he starts getting banged up and winds up missing a few games here and there.
Ridley, on the other hand, has just had some bad luck with match ups, in my opinion, and when the playoffs roll around, I think you'll be glad you kept him.
With all of that said, if you have Rice and Beastmode, I don't know why your 4th stringer really matters.