The 2011 World Cup winner has been playing in Japan having turned down an extension to his contract with the New Zealand Rugby Union earlier this year.
He ruptured a pectoral muscle while playing in Japan that could signal the end of the lucrative stint with the Panasonic Wild Knights.
But he said he was fulfilling a long-held promise to play for the Roosters after he departed league in 2008 - a decision that shocked the sport.
"It was pretty tough (to come back). It's tough, but me and Roosters chairman Nick Politis are good friends, good mates," Williams said.
"He tried to get me when I was an 18-year-old and when I made that handshake with him I knew it would always be the case of joining the Roosters."
Williams sensationally walked away from a long-term contract with the Roosters' cross-town rivals the Canterbury Bulldogs in 2008 to play rugby in France before returning to New Zealand in 2010 in an effort to make the All Blacks 2011 World Cup squad.
He played 19 times for the All Blacks following his debut against England in 2010, though appeared mostly off the bench during their successful World Cup campaign.
The barnstorming inside centre, however, came into his own during the 2012 Super Rugby season, helping the Waikato Chiefs to their first title.
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