First off let me reiterate...I TOLD YA SO...check out my older posts (if you can find them)...I have been saying for moths that Danzo and Tobi are the same person. Though not totally confirmed by the Manga, it is buikding towards that.
I know I am not the first to make that connection, but everytime I state that prediction, everyone thinks it's wrong. Just wait...they are the same for sure.
My guess...Danzo had a bad eye for a lot longer than people think...there are pictures from when he was naruto's age, learning from the 1st hokage, and his eye is covered.
Danzo is not an Uchiha, like Kakashi, he has an injured natural eye. I guess that when Kakashi showed up with a sharingan from Obito, Danzo got the idea to replace his own eye...maybe even scavenged it from Obito's body...but that is pure speculation.
Also I believe that Tobi is NOT really Madara, just pretending since Madara is a missing-nin. I mean the Sharingan would appear to prove it to anyonewho does not believe it.
Tsunade knew something was up with him, thats why she was watching him.
Feel free to disagree as I respect everyone's opinion...but from a storytelling standpoint, it makes sense. DANZO IS TOBI...the proverbial fox is in the hen house...the enemy of Konoha is within it!!
OK haters...lets hear it