While I always understood that grass was hard on Rafa's knees , in part because of the compensating for the foot condition per insoles in the shoes, the low bouncing balls, etc, I always wondered how it could they be any worse on his knees than the pounding of an unforgiving surface like hard courts. As a runner in my younger days, it was preferable to run or grass or dirt over concrete or asphalt.
When he had that streak of early round losses at Wimbledon for five years, (2012-2016) the detractors like to point out that Rafa was done on grass, that his game and body couldn't sustain it. Yet again, once Moya came on board Rafa tweaked his game (serve placement, more aggressive on rallies) so he had 2 SF's on grass his last two W's. I'm guessing it was more his strategy on W that was wrong, couldn't simply rely on raw blazing speed to retrieve every ball as in his younger days, and once he adjusted he did much better.
So if this last foot procedure works, I would agree that grass overall is more forgiving on Rafa's body than the hard courts. That's not to say it's his best surface per his natural game, but maybe the old chestnut that grass is inherently harder on his body could be laid to rest.