It's all relative anyway. Marchisio can basically play any role in midfield except for an out and out winger or fantasista. But with Sissoko, Melo and Poulsen in the team I don't think we should be fielding Marchisio as the lone DM. Maybe Melo can take that left central spot in the diamond midfield, but Sissoko shouldn't be used there because he'll be all over the pitch anyway. His best quality is obviously honing in on the opposition and destroying attacks, so he should be in front of the defense. Poulsen would be his backup, of course.