He doesn't need to know about amaterasu (Though he probably does..he knows about tsukyomi, and they are brother and sister jutsu)
Nonetheless, he knows how mangekyou abilities work.. If he sees a mangekyou user building up chakra in their eyes, he needs only put v1 susanno up to defend
Nonetheless, he knows how mangekyou abilities work.. If he sees a mangekyou user building up chakra in their eyes, he needs only put v1 susanno up to defend