It's not about passing (the fact is that Momo is a decent passer at best, even when we're talking about short passes he's not good) but he's simply given a different role here and not as a clear destroyer like he played under Ranieri.
I'm pretty sure he'd do great now with Roma (or Ranieri) and some other coaches (Mourinho too)
As a clear destroyer very few, if any at all, are superior to Sissoko. In Real he will only have to win the ball and pass it to the closest teammate, which in most cases will be Xabi.
Mourinho knows how to make it work. A Real player (Xabi or a defender) will always run towards Momo when he's about to win a ball, so Momo won't even have to make passes longer than 2-3 meters. In Juve the players don't move well and many times, as we all noticed, there isn't anyone close to Sissoko.