Is there a reason to think he's fallen off? He looked pretty good in week 18.
I tend to discount week 18 games a bit and I think it helped that LaFleur was calling plays and harder to game plan for... but I concede that you might like what you saw in that game.
I do think you have to go back to his tenure with the Raiders and see just how bad he was. There's a reason no one signed him to start after that.
He's also a glass animal which I don't think gets better with age. (issue with the Raiders)
Jimmy barely throws over the middle of the field as a QB which is a problem in itself and with a year +1 from being a regular starter and an already average arm, I don't think he's gonna ever throw a pass again over the middle, he won't be able to drive it (I'm being sorta hyperbolic). (Issue with the 9ers, got worse with the Raiders)
Still, I think you could squeeze a .500 season out of him with the right cast.