Ridley at wide receiver versus learning to play running back is apples and oranges. Wide receiver is by far the easiest position on the field to transition from one level to the next. It is basically run this route or block the guy in front of you. The easy part of running back is the running. Identifying who is coming at your QB in pass protection, even pro RB's and QB's struggle with that. Struggle with it enough, you'll get your QB killed and that's why he was fifth string going into this game. Once he gets that down, the sky is the limit and there isn't anything unusual about a freshman struggling in that department as it's the norm.