I think some of you are confusing some of the scenes.

The part where Tsunade explains how the chakra threads were being severed in each and every cell, and it showed that in like a diagram...that wasn't her explaining the damage to naruto's arm. That was her explaining what the attack did to Kakuzu.

The effect on Naruto's arm is probably the same, but no where near as severe. Which is why Sakura was still able to heal him.