Nadal's fans say everywhere that he was terribly injured at that moment then according to their words no he would have retired in one of his next matches
It really depends how much you think he was injured
Also if he had beaten Söderling in 5 sets it wouldn't have helped his knees
Gonzalez played very well in that tournament, considering how short Nadal played in French Open, I think he would have had a good chance. Federer also would have had a good chance if Nadal played so short, because Federer played very well in the last 3 matches of the tournament. But of course there's the problem of pressure, and also the fact that during the day of the final, the weather made the court slower than previous days.
We all know that the best Nadal would have won, but even if Söderling undoubtedly played a huge match, Nadal was not at his best during that tournament.