Manning was declining as far back as 2010, and it was more than just a nerve issue. The surgery helped him some, and having a pair of big, talented receivers on the upswing helped more, but the basic problems still remained, and they come out when he's under pressure and in bad weather.
He can no longer get away with being less than mechanically perfect on a lot of the throws that perfect mechanics used to be unimportant for. Judging by the rate of decline I saw in his game this season, my guess is that, in the upcoming season, even perfect mechanics won't be enough for him to make all the throws. Barring a repeat of the "look what I found!" that Shanahan was able to pull off when he drafted Davis, next season is probably the last time that Manning will have anything more than a puncher's chance to get that second ring.