De Jong played as the deepest starting midfielder at Ajax. He would collect the ball from CBs or even drop into a LCB position and work his way forwards. Schone started in front of him positionally but was equally or more defensively capable. De Jong didn't really fit into Barca's system which requires a lone DM, which is why he's available. Fred and De Jong would actually be a good partnership imo, as they'd balance each other quite well, relieve Fred of the deep lying playmaker duties, and Fred would provide plenty of running and assistance to support De Jong defensively. A Fred upgrade in time would also be ideal of course.