It would be retarded to blame everything on Sissoko. I don't understand why a physical defensive midfielder is being forced into a midfield role which requires him to have attacking ability going forward. We never saw this in the 4-4-2 because of the width available, and he usually had someone like Zanetti or Marchisio next to him who was comfortable in possession. It's also not a surprise that in the 4-2-3-1, also with width, he did a great job in cutting out the attacks and threats of the opposition and the simple passes to a Diego, Camoranesi or Giovinco created an attack for us.
A midfield of
Marchisio-Poulsen-Sissoko
has no place in football, drop either Poulsen or Sissoko and start Camo if you must use the 4-3-1-2.