I do agree if Pete and Rafa played alot of matches on grass that Rafa could win at some point, especialy if they played at a tune up (much harder to beat Pete on Wimbledon, especialy on the old true grass). However if they only played 3 matches on grass, if all 3 at Wimbledon, and if 1 of the 3 was baby Rafa who was a grass court novice back, and 1 of the remaining 2 Rafa was seriously injured in the hypothetical 5th set, then then I think Pete would win all 3 in that case.
I agree if they played 11 times on clay that Pete would find a way to win 1 or 2. Pete is a very underrated clay court player. His biggest problem on clay is he lacks consistency, but he has the ability to beat a top clay courter, and he tends to get up for the top guys on clay and always make it competitive.