"I feel he'll stay in rugby," Mundine, a former NRL player turned professional boxer, said.
"As great as rugby league is, rugby is the world game. Lots of countries play it all over Europe and South Africa, it's massive. I went to South Africa to watch him play a [Super Rugby] game and they had fans waiting outside the hotel room. It's a big thing. It's the biggest sport over there. It's crazy."
Williams played for the Bulldogs in the NRL between 2004 and 2008 when he defected to French rugby union club Toulon before moving back to New Zealand to play for the All Blacks. He returned to the NRL with the Sydney Roosters in 2013 and 2014.
Williams has played 12 rugby league tests for the Kiwis and has 46 All Blacks caps.
Mundine, 42, who is back in the boxing ring against Tommy Browne on January 17, hasn't ruled out an NRL return for himself.
"I want to have a few fights next year and then my fighting days will be finished," he said.
"People say [an NRL return] can't be done. If I put my mind and soul into it, it can be done."
Photo: GETTY IMAGES. Caption: Sonny Bill Williams still stay in rugby union, predicts close friend Anthony Mundine