Why Sasuke "Already" has "perfect" Susano'o
Right now, most people are under the impression that the perfect Susano'o is some kind of "stage" of the Susano'o. That it has to be "awakened".
That's not true.
Actually, what makes perfect Susano'o important, is not Susano'o itself, but rather, the armor it wears.
Allow me to explain.
What some people currently have in their minds, is that the Susano'o had "stages", and develops linearly.
So "stage 1" would be the ribs, "stage 2" the skeletal structure, "stage 3" the skin etc.
This is a misconception.
Understand that what we called "complete" Susano'o [Due to an original chapter title] is actually the last state of the Susano'o.
In other words? These:
Are the last "stage" of the Susano'o.
Because the next stages, are not "stages" in the sense of being "transformations". The next stages are armor. The "complete" Susano'o is simply equipped with armor.
This was first confirmed when Kishimoto drew Sasuke's Susano'o gaining the armor:
Plainly, the "eyes" we see is the original Susano'o underneath. This was shown again when Madara's "perfect" Susano'o lost it's power when Itachi cancelled Edotensei:
When it was transforming:
And the sheer fact that Kyubi wasn't wearing Susano'o, but rather, Susano'o's armor.
The Susano'o didn't change, apart from it's size. The armor did. This is why I would say Sasuke already "has" "perfect" Susano'o: it's an armor, not necessarily a new transformation.
My theory is that the Susano'o's armor and weapons are the most important parts of the jutsu, while the Susano'o "creature" is basically a means to use those weapons, and something in it's absence can replace it [Kyubi].