goal line backs dont need to be 250 pounds, they need to find the endzone. If rainey can do that then theres no problems. If you have to account for the toss SO much with a guy like rainey, he may just be able to squeeze in up the middle just as easy.
He may get in every now and then, but I hope we don't have to depend on him pounding the rock. All kidding aside, the kid is fast as lightning, but just don't have the girth to be an every down back running up the middle. His niche will be K/Os, punts, and coming out of the backfield. Maybe an occasional gimmick play.