Ok, I can't believe i am spending so much time on this, but here goes.
Let's assume that JWill replicates his YPC average from 2016 (probably unlikely given he's going to be a year further from his injury and in his sophomore year). Then let's assume he get's the same amount of carries that MG got last year. The different in yards would be about 200 yards over the course of the year. Spread that out over 16 games and it's 13 yards a game less that we get from the RB position with JWill, assuming worst case scenario (that JWill doesn't progress at all and MG would have stayed the same).
You're going to say, what about TDs. I would submit that TDs are more of a fantasy football metric in terms of player evaluation. In other words, if JWill is put in on the goal line, he gets some TDs.