Unohana almost made it sound like Aizen's shikai isn't instant, as I can't think of any other feasible way to fight it without the earlier mentioned blindfold trick. Otherwise, those with bankai can whip out their shikai without the evocation command, so it could technically come out of nowhere unless he needed time to prepare.
Also wondering that if it is instant, he could've actually used it on Ichigo and forced him to forget about it. Also leaves the possibility that Aizen technically isn't that strong, but has just fooled everyone into believing he is. We're still missing some steps in his grand master plan, too.
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