Board Thread:Theories and Speculation/@comment-34436338-20180123200352/@comment-734582-20180123212422

Basically Guy was burning up on a cellular level, and while Naruto was able to stop him from dying he couldn't repair all this damage. Tsunade could heal relatively minor fragments in Lee's leg with great difficulty and risk. Guy's leg is entirely shattered and the bone fragmented.

Also, Naruto can't heal his own arm so his healing ability isn't infinite.