Board Thread:Theories and Speculation/@comment-33256714-20171005000302/@comment-734582-20171007132656

Indk21 wrote: So why is necessary to unlock both Mangenkyou abilities to achieve Susanoo as third ability of the Mangenkyou? After Sasuke used Susanoo against Danzo, he felt like every cell in his body hurt. Apparently, Susanoo needs to be stabilized before it can truly be used without eyes. Tayuya once used spirits that drained physical energy, as if they took what they lacked. So, i'd say the same is true for the Susanoo: in order to be stable, it needs to be fed the user's physical energy until such moment that it is complete.

In other words, Susanoo first needs to be cast somehow before it can be used independently.