I'd laugh if that wasn't even Aizen, since he has not used is shikai yet. Ichigo would realize if it were an illusion though, but what if Aizen modified someone hollow to look like him. Kind of a final test before working on himself?
Anyways, Shinji's mistake was already showing Aizen his shikai. The Captains mistakes were not all attacking him at once. They've been slowly increasing their numbers with the stronger ones attacking in the later waves. But Aizen is still managing against the groups. There should have been an all out Bankai-fest with a bit of Hollowification thrown in where applicable.