I think you're forgetting Potts was very good pre-injury last season. He looked hobbled against Rutgers this season, but still had a 40 yd run. He looked much closer to normal again in his long run against NW.
Despite Mo getting most of the carries, Potts was still the 18th leading rusher in the B1G this season.
But we're talking about projecting to next season. Will either Brown or Allen be back for their schools? I doubt it. I would take Potts over Grant, Hull, Berger, Indiana's, or Rutgers RBs. That's nearly half of the conference already.
The other RBs at MD, PSU, Iowa, or Purdue that you think are better were all freshman or redshirt freshman. Allen was a freshman last year. So between Potts/Williams and 3 talented freshman, I think we have plenty of options.