Quote Originally Posted by FullMetalAlchemist
He didn't say that exactly. He said he can't place his affinity in it. So there is no use in useing rasengan when he already has chidori. Remember they are about the same power (rasengan chidori) so there really is no reason to use it. Rasengan is just an incomplete jutsu which it looks like naruto is the only one able to complete it in the end.
In the demonstration he showed that he could only create a rasengan half the size of Naruto's basic one.