That would be a big decline, not at all gradual. He hurt his hamstring prior to Week 1 and he was always on the injury report or missing practice. For a whole season, that's playing hurt.
Some people have forgotten too quickly just how good he was last season - yes the stats were down on the previous year when he led the NFL, but he was targeted a lot less and still basically won us three games with his key plays.
This has been a down year, but like Edge has mentioned he has played hurt all year and is still easily our best CB out there. If he pulled in those two drops he would have had four INTs for the year - not good by his lofty standards, but I doubt we'd even be having this conversation if he did.