Board Thread:Naruto Discussions/@comment-26993711-20160319001508/@comment-26894845-20160319154315

As good as the replacement zetsu arm idea is, it can't be it. When he comes back out of the dark, he puts his mask on with one hand, and the other arm he puts behind his head, revealing it is still there. Also, the flames had already spread to other parts of his body. I am convinced that it must have been izanagi, and that the thing Itachi didn't know was the collection of spare sharingan.