It's a trite expression, but it seems apt here: De Gea is an a great shot stopper, but his command of the box seems lacking. This could be because he works behind a constantly rotating defense, some of whom are inexperienced. Maybe if he played behind the same back four more often he would make less mistakes and be more commanding of his space.
He's still awfully young, still needs to master English, and definitely needs to play behind a settled back for (who in turn needs to play behind a settled midfield for a change). DeGea has all the attributes to be a top, top keeper for many years. There will be bumps in the road, for sure, though.