I think it's fair to say that the coaches staff completely botched this situation. According to the OWH, over a four-game stretch in the Big Ten after Rex went down, he only recorded 10 carries while Ameer recorded 95. There's no reason to distribute the ball that unevenly, especially considering how fatigued Ameer looked at the end of the season.
Rumors of injury to Heard's leg, though. I have no idea what's true. Everyone is banged up that time of year. You'd think that if he's able to get 10 carries he can get 20, maybe 30. That's a heck of a lot less wear on Ameer, and better "roster management" (at the risk of irritating BRB, sorry).
This has been my point all along. When you have as many good RBs as Nebraska did last year (and 2010), why ride one so much? Keep him fresher by running the subs more. It has the dual positive effect of making the backups happy that they are seeing more playing time and keeping the starter from taking more hits than they need to. The perfect example: Arkansas State game last year. That game was never really in doubt with a 28-3 halftime score. Yet Abdullah had 30 carries, Cross 7, and Heard 10.
Michigan: Abdullah 24 carries, Cross 2, Heard 0.
Michigan State: Abdullah 22 carries, Cross 0, Heard 0.
Penn State: Abdullah 31, Cross 8, Heard 3.
For the people saying that Heard's YPC stat was high because he was playing "garbage time". Well, that's bogus too. Take the Iowa game for example, with a 13-7 final score, there was no garbage time. Abdullah 14 carries 50 yards. Heard 4 carries 46 yards. Sure, maybe Abduallah wore down the defense some for Heard to break off bigger runs... but that's exactly the point. You want to be using multiple backs and keeping everyone fresh, right?
Don't take this as me saying Abdullah isn't a good back, I think he's a great back. But to say there was no place for Heard to get more carries last year seems crazy.