Board Thread:Versus Debates/@comment-26005711-20160124210949/@comment-26537507-20160125204242

Sage Mode is an absolute last resort for Minato since it doesn't fit his fighting style, so while the genjutsu scenario is definitely possible, it's also quite unlikely.

That's more than just putting two and two together. Consider how Naruto worked it out - he got told only a really strong wind attack would work, tired RS but it's effect didn't last long enough, started considering other tactics, remembered abstract story about Raikage fighting the eight tails and ending up with a scar he won't talk about, figures out he won't talk about it because he's embarrassed he inflicted it on himself, uses Rasengan to deflect.

To go from "Hmm, nothings working." to "I'll just deflect this so he can injure himself like he did before." is too great of a logical leap for someone to make unaided. Sure, given enough time Minato would work it out, no doubt about that, but in the space of a battle against the fastest and probably strongest opponent he'd face (excluding Madara of course), not likely.