The thing that I don't like is not people saying someone was injured, it's people assuming and insisting they know for a fact that the match would have gone the other way if not for that. Upsets do happen. Nadal at Wimbledon was not likely to lose to Rosol but then again Rosol played a great match and Nadal has often been troubled at Wimbledon before by low ranked players. He won 5 set matches against Robin Haase, Petzschner, Youzhny, Soderling and Kendricks - the first 2 of those mentioned were in 2010 when he won Wimbledon and 2 other slams. It's not absurd to think one day someone would actually win one of these 5 set matches vs him, and Rosol kind of had the lack of respect that maybe you need to beat a top player. Nadal had just won RG supposedly recieving treatment so he could continue playing, he then decided to play doubles as well as singles at Queens.. it seems whatever injury he had he was managing well enough to play even if he was heading for disaster.
At RG in 2009, again it's unlikely that he loses to Soderling. But you have to figure in 8 years of play, one year someone is probably going to beat him even if he is fit. He might have been below 100% but in previous matches he looked good and still looked a certainty for the title. Soderling probably played a one in a million match. Another day Nadal would have kicked his arse in the exact same shape, but that day it didn't happen, partly because the court looked faster, partly Soderling played his best ever match and hit masses of winners, partly Nadal wasn't competing at 100%. But not every time you play will you be 100%. That's different from being unable to play.
Federer lost to Djokovic in Australia 2008. I think he was not at his best, still a bit below par and out of practice. But I think he would have lost anyway. Maybe a bit closer, but saying he would have won that for sure is absurd.
Maybe the matches would have had a different result, but maybe not. There's not much point debating it unless a guy retires while significantly in the lead and clearly dominating but is not able to carry on playing. You can also apply this to things like wind/roofs/rain/darkness. Even though it may have a bigger effect on one player's game it doesn't mean the result would be different. Something like wind or darkness just makes a game more error prone and adds a random element. It may favour a safer player, but that doesn't mean they would not win anyway.
How to clarky - work out most or only realistic outcome, claim the opposite
Federer, Djokovic, Delpo fan (also like Nalbandian, Dimitrov, Tsonga)