Board Thread:Naruto Discussions/@comment-1500702-20190701205352/@comment-9782468-20190703173903

@Omnibender, the other Hokage were impressed that he already placed his kunai when getting to the battlefield. The fact that we see his kunai doesn't mean it was always there, it (in my opinion) just means he made it to the battlefield and placed his kunai fast enough to stop the disaster and prepare to create a barrier around the Juubi.

After all, there would be literally no other reason for Minato's kunai to have been there other than to just have placed it.