User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-104.7.80.176-20150212132646/@comment-24502975-20150212145237

Good question. They were in the midst of battle in volume 18 so they didn't pay attention to that. Or if they did, I can't find a section of translation that mentions it/makes enough sense to realize they're talking about that. But Cao Cao did mention he no longer has the Medusa's eye, so since an eyepatch would be instantly recognizable I'd say he got a replacement normal eye/prosthetic one.