My guess is that McD is heavily involved in the RB selection process. He said, last season, that over the years they (I believe he was referring to the coaches he worked with) had identified certain traits that tended to be successful in their system and that he had spoken with the scouts about what he was looking for. My guess is that if they identify a RB that fits that profile, they'll take him where ever they need to.Grier thinks he can find RB steals in late rounds and never can. I question his ability to put an OL and run game together.
The thing is that running in a zone system is a little different. You don't just run to daylight. You have to anticipate and see the flow. It's a bit like what Tua does with his anticipation throws. And reality is that not everybody can see that. Chase Edmunds was a very good back in AZ, and I have no doubt he put in the work here, but he never did figure out how to anticipate the lanes. That makes the evaluation tough.