Board Thread:Naruto Discussions/@comment-4682609-20140806081616/@comment-1043414-20140810191915

Technically, his left eye never had the Sharingan to begin with. It was always Obito's. But considering that Kakashi grew up and developed with the said eye, even awakening the Mangekyo form, it does stand to reason that he might have integrated some of his essence. Danzo, who also had a "borrowed" Sharingan, couldn't maintain a single eye for very long with each usage, ultimately needing to get a new one periodically. Kakashi's however never showed any signs of permanent damage.