Board Thread:Naruto Discussions/@comment-24373005-20140811055011/@comment-24912592-20140811074207

There is a different possibility, though.

It is said you need a close relative's eyes. Kishi never got into it too much. Think about it... Close relative... Why? Maybe for a biological reason? So that the body won't reject the implant? Which means, if Kakashi's body already accepted the implanted eyes... He might have an inferior EMS now. Inferior as in never goes blind, but lacks the strength of Kakashi's personal Mangekyou neurons.

Well, that's just a possibility, of course.