Lets take a look at den Dekker's season:
Quick summary: In April, he hit everything, hit for some gap power and never walked. In May, he walked more, but hit for less power. In June, he's walking like a champ, striking out less, and hitting for power again.
I don't want to read too much in 51 plate appearances in June, but the pattern to me shows a guy who thrived early on ability and athleticism, was forced to make an adjustment when the League and the randomness of baseball caught up to him in May and now is a better, more well-rounded player in June.
These types of adjustments, to approach and mechanics, are much harder when a player is also dealing with moving to a higher level of ball, in den Dekker's case to AA. There were plenty of fans who were crying out for den Dekker to move to AA in early May. However, he's clearly used the extra time in advanced-A well.
It's a good thing that den Dekker has learned how to take his walks and hit the ball over the wall. Now, AA beckons, a man hitting .304/.360/.512 for the year.