The running back position is not an easy one to play in the NFL, either on the field or in the marketplace. No other player takes more of a beating in either dimension—and the two are not unrelated. Part of the reason that running backs struggle to capitalize on and maintain their status in the open market is precisely because of durability issues on the field.
This offseason has already been unkind to running backs, with guys like Dalvin Cook and Leonard Fournette having their contracts terminated. There had been much debate over whether or not Joe Mixon of the Cincinnati Bengals might join them.