Board Thread:Theories and Speculation/@comment-34436338-20180123200352/@comment-33286580-20180123232211

Gai's leg had already started turning to ashes by the time Naruto got to heal him. and if you actually read the chapter when Gai used Night Gai, you can see in the previous page that Madara had his staff held up in the position where Gai's kick landed. that meant Gai was in direct contact with the TSB.

no damage by the TSB can be healed. so could be the case here. even Naruto with his healing power could not undo the damage done by TSB.