The 29-year-old was part of the All Blacks' triumphant 2011 Rugby World Cup squad before switching to league but has now switched back - indicating his desire to defend the trophy in 2015.
And Hansen believes that Williams' natural talent combined with his previous experience of the sport means the powerhouse will have little trouble re-adapting to union.
"I don't think he has as many challenges as the first time he crossed from league to union," said Hansen.
"The first time he came he didn't understand the rugby culture, so he had to learn the game from a zero foundation.
"Now he has played 19 Test matches and that tells you he has been good enough and done enough to be in the All Blacks.
"He is a reasonably handy footballer and you don't forget the things you learn. He's excited about being here and he's fitted back in really, really comfortably.
"What we will see from Sonny over a period of time is him getting better and better as a rugby player. He's a freakish athlete and he could become one of the very, very best we've ever had.
"For us, what matters most is that he's back and he is understanding how we are playing now as we are playing differently to when he left in 2012."