Board Thread:Naruto Discussions/@comment-36221354-20180717114536/@comment-34938099-20180805065844

In my opinion, you don't exactly need a Sharingan for awakening the Rinnegan. It's just that evolving Sharingan to Rinnegan is one of the many ways you can acquire the Rinnegan. As far as we know, it is not  compulsory  that you need the Sharingan to get Rinnegan. Yes, you can get Rinnegan through Sharingan but it's not the  only  way you can acquire it. Because Otsutsuki like Momoshiki, Urashiki, Kaguya and MANY other Otsutsuki awakened THEIR ocular powers ( I.e Rinnegan, 6-Tomoe Rinnegan, Rinne-Sharingan ) by eating Chakra fruits from one of the God trees across the Universe / dimensions, most likely.

Also, at this point, you can't even say that you need Hagoromo's Chakra to get the Rinnegan. Because many others have awakened it WAY before Hagoromo. So, you can't call it 'Hagoromo's Chakra' or 'Hagoromo's Power' now but most likely the power of the 'Chakra fruit' or the 'God Tree'.