Board Thread:Theories and Speculation/@comment-33485854-20180625233306/@comment-1512913-20180629081232

Madara's eyes because Rinnegan because of Indra's chakra inside of his body mixing with Asura's chakra inside his Hashirama transplants.

Black Zetsu said he tried incarnates on both sides to awaken in. A Senju Asura incarnate (Hashirama for example) wouldn't need to get Madara's or someone else's Sharingan to get Rinnegan. Any flesh from Madara's body would do for Hashirama to awaken the Rinnegan, the eyes are not necessary.