JMM ducks low and swings wide when countering, meaning he requires distance to really land those counters. Castillo is an expert at smothering an opponent and is difficult to move once he gets his weight on you. JMM would have to do alot more leading and moving than he's accustomed to doing, and almost all of his work would be done off of the back foot. If he tries to stand his ground, Castillo wears him down with ease. JLC wasn't all that easy to hit, either (wars with Diego notwithstanding), and was a pretty good counterpuncher in his own right.
By the way, as with the trend among other Marquez fights, I see him being knocked down on three different occasions against Corrales but being able to do quite some damage and force a stoppage himself. Ala Katsidis fight, but his face would resemble the last Pac fight.
Corrales would whack Katsidis out brutally, a whole different level of pain. I do agree with you though, I think JMM could get through it.