'Let’s start with the concerns. Jackson turns 34 on Feb. 20, and that raises questions about just how effective he’ll be through another season. It doesn’t make a whole lot of sense for the Bills to make a major investment to retain Spiller, whose durability is an issue. If he departs, the Bills would need to find a replacement in the open market, which includes some big-name pending free agents in DeMarco Murray, Frank Gore, Justin Forsett, and Knowshon Moreno, or the draft.'