Board Thread:Naruto Discussions/@comment-26210090-20150315213729/@comment-25771935-20150316041338

Also don't forget that in Lee's case he had a couple of fragments in his spine compared to Guy's entire leg being shattered, so healing him with the same jutsu would most likely be out of even her reach. As far as replacing it, the only thing that I could think of is what everyone else is saying-all the other ppl who used white zetsu limbs were descendants of the sage except danzo, who had to keep his arm sealed until he needed it.

And I don't think it's bad writing, more like a noble kinda more believable way to retire Guy.