Rivals lists the best RB in the portal as Wake Forest RB Christian Turner, who played his first three years at Michigan.
So in 2020, Michigan had Hassan Haskins, Zach Charbonnet, Blake Corum, Christian Turner, and Chris Evans, and went 2-4, finished #11 in the Big Ten in rushing.
I think one of the more interesting what-ifs, is that if Michigan hadn't opted out of the end of the season, and instead played (and presumably lost by 50) Ohio State, would Harbaugh have even had the option to return?
Not playing that game, may have delivered Michigan these back to back Big Ten championships