User blog comment:Omega natsu2/Chapter 390: The Story of a Boy, Predictions./@comment-98.26.119.147-20140620074745/@comment-2214433-20140620080408

Yeah, that sounds quite reasonable and more or less what I was thinking, I guess the only problem is that it seemed rather abrupt. Let's face it: We all knew Minerva would be eventually redeemed, but I guess nobody expected it happen so fast.

This leads me to think that Mashima is saving Erza for something else in this arc.