IMO, Peyton Manning is done for the year.
I've had Plantar Fasciitis. It's a pain in the ass, and takes a long time to heal. Dropping back to pass will aggravate it, every time.
What makes it such a pain, is that it's not such a bad injury that you're gonna sit at home for a month with your feet up, but, when you do walk on it, you're not allowing it to heal and it ends up being a several month injury.