Board Thread:Naruto Gaiden/@comment-27829630-20151230183737/@comment-25122992-20151230184623

It was likely transplanted to him by Orochimaru, although the source remains unknown. Because of his ability to adapt any genetic material transplanted to him, Sharingan essentially became the actual part of his body, so his mastery over it is at least very close (or even equal) to that of a true Uchiha.