Muu can't just sense the Chakra built up for Amaterasu during a battle. People are confused as they take Juubi's Jin as a measurement. Staying next to Itachi and feeling a pressure isn't the same as actively sensing an attack that happens in an instant during a fight. Kabuto is also a sensor in Sage mode and has a great sense for danger. Yet Sasuke fired his Amaterasu instantly before his feet [
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]. You don't even need sensing to react at Amaterasu, eye contact is enough. But reacting and dodging are two different things. Neither Muu nor Oonoki can dodge it.
And btw,
anyone is aware of the Uchihas strong Genjutsu. But this knowledge alone never helped someone to avoid them in the long run, as the Uchiha would eventually catch the victim nevertheless with prior feints or distractions which will force the victim to look into his eyes. The whole chatter about a Genjutsu counter with simple prevention of eye contact is BS. Sakura already mentioned how hard this matter would be. Not to mention that the victim will have simply no chance to avoid eye contact in the long run, if Itachi will pressurize him, use feints and distractions. Naruto told himself twice not to look into Itachi's eyes, yet he nevertheless caught him in a Genjutsu.
Oonoki will avoid eye contact and keep distance, Itachi sends out a bunch of crows to attack him and put him into Dusk Crow Genjutsu. Crows have always proven to be a perfect distraction.
MS Madara soloed Oonoki and Muu easily, he played with them, never went serious. And the gap between MS Madara and MS Itachi simply isn't that big to say, that MS Madara shits on them but MS Itachi loses.
Itachi and Kisame will take this without much effort.