What has changed is that after 2012 he can't flex his knees the way he could anymore. He's pointed it out several times now. He can't adapt all that well to having to flex to hit the ball anymore, not on a consistent basis, after his knee issues back in 2012.
Yes, I've heard him say this... but can't say I find it a wholly satisfactory explanation
Flexing the knee would be most important for low bouncing surfaces like grass - but if Nadal's was in such poor shape as to outright be the direct cause of his troubles on grass, I would think it would have adversely affected him on hard courts to a
much greater extent than it seems to have
2012 and on, he's 13 - 8 on grass (and that includes a 250 tournament win in Stuggart)
At Wimbledon, it's 8 - 5. Best showing 4th round
Prior to this year, when as you say he was playing well, that slips still further to 10-7 overall and 5-4 at Wimbledon. And all 4 defeats to players ranked outside the top 100
That's atrocious - and not just by his high standards
By contrast, he's 159 - 43 on hard courts over the same period (121 - 35, sans 2017). 2 US Open titles, 2 Aus finals (excluding 2012 edition when the knee was presumably fine)
Respectable results on the fastest and/or lowest bouncing venues like Shanghai, Paris, Basel, London, Cincinnati (multiple finals and semis - as well as his only title at Cincy)
I'm sure the knee issue has played a part, but if it was the primary reason, I doubt Nadal could have done as well as he has on other, not dissimilar surfaces
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Some of it's probably mental, too
More than most, Nadal both thrives on confidence and can seem crippled without it
Do you remember the thorough thrashing he gave Andy Murray in 2011? It was glorious, commanding stuff... of play and demeanour, he looked like the Lord of the court. How about 4th set 2007 final? I haven't seen Roger Federer made to look so helpless on grass like that, not before not since
Since 2011 though, on grass Rafa mostly looks like a turtle trying to decide whether to stick it's neck out or not, leaving the play on the other guys racquet
He was very good this year, it should give him some much needed belief.... I wouldn't be surprised if he has a deep run or two left at Wimby