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Given Jackson's age and injury, as well as Spiller's ability to produce, it's a fairly safe assumption that the Bills will work Spiller into their offensive game plan more in the early part of this season than the last.
In 2011, when Jackson was healthy, he was the workhorse and Spiller was a complete non-factor.
So long as Jackson remains so, he remains the odds-on favorite to receive the majority of touches in the Buffalo backfield.
However, we're still going to see a lot more of a platoon situation in 2012 from the Bills at running back.
In fact, there may well be weeks where a perfectly healthy Jackson is out-pointed by Stiller in fantasy football—especially in point-per-reception leagues and in games where the Bills are forced to abandon the run early, as Spiller is a much more dangerous receiver.
A healthy Jackson may not make Spiller worthy of a draft pick as a weekly fantasy starter at running back, but he could serve as a more-than-viable bye-week fill-in or "flex" starter, especially in PPR formats.