Board Thread:Boruto Discussions/@comment-28432164-20160514001744/@comment-734582-20160514164331

Naruto didn't get rusty. it was just Kurama doing kurama things and mocking Naruto. He mocked Naruto in front of the other Jinchuriki, he mocked Naruto in front of the Hokage. Naruto was never in any danger versus the Shin, and probably hadn't experienced any form of injury whatsoever so being pierced by a sword wouldn't have been pleasant. But even that didn't actually injure him, since he just went on like nothing happened.