De Jong will leave Ajax for Barca at the end of this season.
Van Gaal said, "He's a player with exceptional talent but he's still developing and improving his qualities."
The Dutch coach thinks the same about Ajax defender Matthijs De Ligt.
"I don't know them both personally, so I can't judge them. But I see how they speak to the media and they are doing well. Better than Van Gaal did back in his day!" he joked.