My guess is that Obito broke. He didn't die when the rocks fell on him and spent x amount of days waiting for Kakashi, Rin, and Minato to dig him out. When they didn't he got pissed off. Somehow he crawls out of his hole where an aging Madara finds him. Madara uses the power in his possession (just a really good medic that he knows) to heal him completely, but since he doesn't have his second sharingan he uses his mask to cover the hole where his eye should be.
Tell me have we seen Madara use the MS? All I remember seeing is just the regular 3 tomoe sharingan.
Well anyways, Madara sees the resentment in Obito and starts training him to continue with his mission to destroy Konoha. So he teaches Obito everything he could, before Obito either kills him, or Madara dies of old age.
Sasuke doesn't know Obito from Madara so he's not a good source of information on the subject. And the last body in Kabuto's coffins was Madara's. Obito freaks out because now there is at least one person who knows that he's not really Madara and the mystique that comes from the name is lost.
Bullet Tooth Tony: You should never underestimate the predictability of stupidity.
Reporter on Titan Maximum: Sasha is there any connection between your poor album sales and the fact that it sucked?