http://sportsillustrated.cnn.com/vau...95/3/index.htm
Each season from 2003 to '06, a different back led the Broncos in rushing, and each gained at least 1,000 yards. That established the economic blueprint for managing the position: A team can succeed, sometimes even win the Super Bowl, without a great running back, provided there is ample talent at quarterback and offensive line.
Hence management will economize at the position, moving out highly paid, established backs in favor of cheaper up-and-comers or journeymen.
"I am of that mind-set," says Chiefs general manager Scott Pioli, who came over from Bill Belichick's Patriots regime in 2009. "That special guy [Payton, Smith, Dickerson] has always been rare. Generally speaking, you need a good season out of the position. In New England, Antowain Smith was not a great back, but he had a great season [rushing for 1,157 yards in 2001, a Super Bowl--winning year]. Corey Dillon was not a great back, but he had a great season [1,635 yards in 2004, another Super Bowl year]."
It is also a guessing game. Every running back comes stamped with an unknown expiration date. "You don't know how many times they've been hit in high school, hit in college," says Pioli. "Or how well they've taken care of themselves."
Shanahan, who orchestrated the revolving door of rushers at Denver in the early 2000s, says, "There are places on the field where you need a great player. I don't think running back is one of them. I look for overachievers."