Board Thread:Naruto Discussions/@comment-34569788-20180504183427/@comment-6769189-20180506085657

Thekillman wrote: Naruto has the spirit half of the juubi (in the form of 9 tailed beasts). Hagoromo manipulated that power to bring forth SPSM. Sasuke had Zetsu cells, Hagoromo manipulated that to give him Rinnegan without losing MS. He did something similar by splitting the Juubi, so it's well within his power. Total speculation. There is nothing to suggest that Hagoromo did anything other than grant those abilities unto the 2 of them. It is questionable whether or not Tailed Beast chakra is necissary for 6 Paths Sage Mode.

It is also possible that the Rinnegan is within an Uchiha eye and is reactive to Hagoromo's chakra, only activating if it is empowered by 6 Paths Chakra.