I'm not too worried about Miller leaving. The kid has Elite speed and has done well this season, however, he is hesitant to hit the hole at times and shuffles his feet too much.
I think NFL teams will drool over his breakaway speed, thus, he will go first round. However, with the addition of Duke, having EC and MJ back there, I think we will be fine in the running game. I honestly expected Miller to have more big games this year than he has. I know he rushed for 1000 yards, but with that type of talent he could have easily had 17 - 1800 yard season.