Board Thread:Naruto Discussions/@comment-25977287-20150704114842/@comment-4912646-20150705014840

It's possible to gather natural energy without automatically entering Sage Mode. We saw this with Naruto's shadow clones that were left at Mount Myōboku and when Madara absorbed senjutsu chakra from Hashirama while he wasn't in Sage Mode. So perhaps Minato had already gathered the natural energy but didn't enter Sage Mode until the last moment because, as he himself said, he can't maintain it for very long.