Seems like too many of us like to give #9 a mulligan on his play too often. If you get the stats and the glory, you should also be the goat when its deserved.
That being said, two things I've noticed this season:
1) His inability to hit a deep route in stride. Just about every deep shot, the receiver has had to pull up short when he had his defender beaten.
2) His inability to hit targets on short timing routes. In the past, he had the vision (and confidence in his receivers) to place that rock where he knew the receiver was going to be in the pattern. Now he's holding the ball longer and the window vs the defender closes faster.

Not sure if these are indicators of 'decline' per se, but his mechanics are definitely off.