Pierce has got a problem with his energy level, and to me it looks physical not mental. I blame the weird knee infections, which he also dealt with last year or the year before....He said earlier this year he "feels old" or "feels like an old man".
So now, when he say he feels good, he really means good for an old man. And to me it looks like he only feels good after a good long rest, then he is energy level is quickly diminished again by small amounts of play.
I have zero basis for the following (except the knee infections), but it seems like a Lyme Disease type of thing......if you feel like you have mono but don't, one can start to suspect something horrible like Lyme, which, unfortunately, is hard to test accurately for (lots of false negatives and controversy, etc..)
A classic Lyme symptom is blown up knees (infection). So that worries me. I hope he hasn't done a lot of vacationing on Martha's Vineyard or the Cape...
I don't believe KG is in decline: I think he's simply playing on a different knee than he played on his whole career, and it ain't as good as the last one....