The beheaded corpse was shown miles away to a point you can`t posibly see if the body is actually missing a head, while we`re at it, when he got decapitated the camera never actually showed that his head was deatached.

A "real" Rasengan would be far far faaaaaaaaaaaar messier than a clean decapitation, trust me.

The only reason a person got decapitated in the manga is because he is an inmortal so Kishi and get away with it, so it`s not that bad.