Board Thread:Theories and Speculation/@comment-1512913-20161119171442/@comment-4912646-20161207183857

There still has to be more to the whole Susanoo without eyes thing that we don't realize, maybe some sort of additional requirement. Remember that Sasuke nearly went blind from overusing Susanoo, and it's pretty laughable to think he could have deactivated his Mangekyō at any given time and kept using it. Not to mention the fact that aside from its debut (where the colour in Itachi's eyes had faded and their form wasn't really clear), Susanoo was consistently used with the Mangekyō (or Rinnegan in Madara's case) in every appearance up until Madara's no-eye usage, after which Sasuke suddenly stopped using it with his Mangekyō as well.

It's easy enough to theorize that Kishimoto changed his mind about how it worked later in the manga, but in-universe there's no explanation for all of the prior uses involving Mangekyō. Even in Madara's case, when Ōnoki first sees Madara's Susanoo, he takes it as proof that Madara must have activated his Mangekyō, further establishing the link between him. It's a really weird bit of dialogue to include if the Mangekyō is only required for the initial activation.

The easiest explanation would be it can only be used without eyes once reaching a certain stage in development, perhaps the final Complete Body form. Though that only works if you assume Itachi still had his Mangekyō on during Susanoo's initial usage and its pattern had just faded away, or that he unlocked the final form and just didn't use it on-screen, which seems incredibly unlikely.