"By the 2005 NFL Combine, Clarett was too slow to turn any heads, and his only blessing was that Broncos coach Mike Shanahan was arrogant enough to think he could save him. Shanahan thinks the system makes the back, instead of vice versa, so he picked Clarett in the third round and found out the hard way."
I give the Broncos credit for being able to plug almost anyone into their system and make a good running back out of them.
But as Clint Eastwood said in Magnum Force, "A man's got to know his limitations." Shanahan discovered his the hard way.
By the way, I recommend the article on Clarett, who is a classic case of someone throwing their life away.