Spiller didn't put fear into defense in Buffalo so I don't see him doing that in Green Bay.
Spiller's averaged 5.0 per carry over his career. His 1703 yds. from scrimmage on a mere 264 touches, with 6.0 per carry, was one of the most productive/efficient seasons from a running back in the last decade.
Given the fact that Buffalo has fielded poor to mediocre passing games in recent history, an opponent not game planning for a healthy Spiller would be, in a word, stupid.
The questions are many...cost, health, scheme fit, compliment to the current players.
I don't see it happening, of course, but it's an exercise of the imagination in what a player like that would bring to the offense.
Imagine for example, that Cobb does not get re-signed. There's not slot-type receiver on the bench. To compensate for such a missing cog in the machine, a player like Spiller might be used as a third down back, a change of pace from Lacy, and a guy who could motion to or line up in the slot...as he was used in Buffalo when paired with Jackson. 20 snaps per game, with a different look that adds a dimension to the offense.
It's not a bad idea per se. But probably not a good enough idea.
