In my vision, Milan's two good formations for this season are 4-2-3-1 (with Kaká) and 4-3-3/4-3-2-1 (without Kaká).
In a 4-2-3-1, there's little doubt his ideal role is a double-pivot with Montolivo, which is a partnership with potential to bo one of the best around.
However, it looks more trickier when we look at a three men midfield formation. He's been playing anchor, and I think he's been doing well so far. But our midfield structure this season is different from the one we had during our best moments last season. Montolivo was usually the central man in this trio, playing as a regista with Flamini and Muntari alongside him, and he found his best football at this role. More than that, Montolivo was key for Milan achieving some great series of results.
This season, De Jong has taken his position. Even though he's doing very well, I think we lose more not having Montolivo there than having Nigel at that spot. But they can still play together. IMO, De Jong should take Gattuso's role when playing in a three men midfield. I think he'd do very well at that role, since he's great recovering balls, and could have more freedom to tackle further up the pitch when the opportunity presents itself. And since he's more technical than Gattuso, he could offer more in build-up in advanced areas, which was something Rino struggled with at times.