If anyone has doubt over how valuable Sissoko was last season, just go back and watch the first leg v. Ajax.
For the first 30 minutes, Ajax ran circles around us, it was embarrassing really. Then, when Jan got injured and Sissoko came on...the game completely flipped on it's head. Yes, we lost, but we probably would've lost 3-0 without Sissoko.
He's not Zidane, but he was phenomenal last season.