Healthy Itachi isn't fanfiction...
You must be registered for see links
His final decline in health was rapid enough for Zetsu to have been previously unaware of it.
First of all, Zetsu didn't know that Itachi was throwing the fight.
Secondly, Zetsu didn't have in-depth knowledge of Itachi.
Thirdly, Tsukiyomi/Amaterasu makes the user gradually go blind, Susanoo makes the user gradually die. It attacks the body and makes you cough up blood, it's also stated to drain away the user's life.
By the time Sasuke unlocked his second MS his vision was starting to blur, by the time he mastered Susanoo he could barely see.
Itachi
cannot have mastery of the MS and be healthy, it's contradictory to canon and thus fanfiction.