Board Thread:Versus Debates/@comment-31338621-20190129080042/@comment-31338621-20190206053749

AsianReaper wrote: Tsukuyomi has never been used in the presence of a sensor (unless I've forgotten an instance). Both are Mangekyo Sharingan techniques and both require build up and both require a whole lot of chakra. It makes sense that it could be sensed.

In any case, Kakashi had plenty of time to ready his team against Tsukuyomi before Itachi actually cast it, and he's not a sensor Kakashi knew about Tsukuyomi.