Originally Posted by Lightbringer
Stabilizing the chakra is the final advent for actualizing the "True" Susano'o, and as you presume is a skill. The complete act emerges from the donor's eyes received to create the Eternal, which supplement the keepers own ocular powers, and must be handled to a certain point 'x' before being able to be reformed into the True Susano'o.
Cut short, Sasuke presumably must handled the appendages that are birth from the Eternal before his True Susanoo is realized. Sasuke must find out the functions of his recently acquired dōjutsu before that power can be transferred and create the True Susano'o. There's a precondition that allows the Susano'o to transform; which is unknown, but could be reason to Sasuke or the receiver mastering the powers of the donor's Mangekyō.