I think he wasn't even on their radar until after the surgery. Aogiri had people looking for "someone with Rize's scent", meaning that Eto was interested in whoever Kanou had experimented on. But then Tatara decided he was no good, and they kind of tossed him to the proverbial wolves. Whether he was really tossed aside or simply being forced to toughen up the hard way is up in the air.
But after he got strong, then Eto was definitely curious about him.