The point were the spark leaped over and caused the fire was when Neuer played Algeria in the World Cup. People were divided in calling him either lunatic or genius, and he went under public observation. He underlined his good performances in the following year and now everyone actually agrees that sweeper keeping is more than just sidepass accurately.
Even Donnarumma said in an interview that his role was inspired by Neuer. Not saying Valdes wasnt great at what he did, he even did it before Neuer was known for it, but Neuer was the one who sold this role. He isnt the best gk of all times, surely not, but if w'd be talking about the most influental then I would say yes and i'm not the only one there.