Nalbandian is a good player on clay, but not better than Federer. If he was so great on clay, he would have 1 SF, and 2 finals at the FO. He doesn't. The best Nalbandian has at the FO is SF. Nalby fans need to stop harping on his groin injury. So what? Who cares. Staying healthy is a part of the game, and being in top shape and healthy is especially important on clay. If Nalbandian can't cut it, he should retire. Back in the day, they didn't even used to have injury time-outs. If you couldn't serve or be ready to return within the required time, too bad, you forfeited.
Federer was already playing well against Nalbandian before his injury anyways. The fact of the matter is this: Nalbandian lost. And there are many players that had a 1-up on Federer, then he came back and crushed. It is far from certain -- and indeed, quite unlikely -- that Nalbandian would have beaten Federer at the FO had he not had an injury.