The biggest problem with this idea is that Mannion for all intents and purposes hasn't taken a meaningful rep in practice since training camp. Foles has gotten the lion's share of reps every week with the game plan. Keenum takes nearly all of the "scout team" reps to stay sharp. That laaves Mannion on the outside looking in.
So unless and until they move him up on the depth chart, he's unlikely to be given any consideration at all. This is the pure definition of a red-shirt year for Mannion. But IMO. He should come into 2016 with a legitimate chance to compete for the job.
It's no coincidence in my mind that Foles was given a two year deal. Sean Mannion could very well be the guy moving forward.