It is from his face transplant. He is really the 12th Imam.
I know your comment was meant to be slightly sarcastic, but you may very well be right on target, in a sense.
The differences between the 12th Imam and the Mahdi. Call it what you want, but the Mahdi can be declared anyone who brings chaos to the world to conquer it as a result and to bring about the new age when Islam will rule the entire world.
The 12th Imam is the resurrection of a 5 year old hierarchical Imam who disappeared right after the funeral of his father, the 11th Imam. It is believed by the Shia that the 12th Imam will rise and lead the armies of Islam to victory.
The stunning thing about the whole Mahdi/12th Imam prophecy is how 180° it is but parallel to the prophecy of the Revelation in the Bible.
The important thing to keep in mind is that by 160 AD, the Christian version of the Bible was widely distributed and the faith was growing rapidly. The visions of Mohammed did not occur until 610 AD, from which he created the faith of Islam. Either he was seeing a counter vision or he knew he had to do something about the rise of Christianity and created Islam as a counter culture cult bent on destroying it.