Well, explanations that occur before the movie hold 1. if you disavow the story of the prequels (not that I'm saying I do... entirely) or 2. the answer is left ambiguous. I've never read the novel, so I can't be entirely sure of it, but I don't think it ever came out and said that that's what happened to his face. But starwars.com (sad, but, yes, it is the official site) always said that his face was disfigured by using the dark side of the force. And the fact that he didn't say something about it to Anakin right after it happened kind of works in favor of him being injured. Think about it. If you see someone get stabbed in the arm with a sword and (duh) develop a messy bloody wound, the victim wouldn't say afterwards to the witness, "Well, I got stabbed in the arm with a sword," because it's obvious what happened. See what I'm saying?