Williams, whose abrupt career changes have enraged fans and blindsided officials in both codes, had been widely tipped to turn his back on Australia's National Rugby League (NRL) for a second stint with Super Rugby's Chiefs in 2014, and a view to a second World Cup appearance the following year.
But the New Zealand Rugby Union (NZRU) announced on Friday they had given up on coaxing Williams back next year, shortly before the Roosters trumpeted they had retained the brilliant 28-year-old who helped them seal the NRL title on Sunday.
Rennie, who coached Williams in his 2012 championship-winning side, said the former All Black centre would still be back in 2015 in time for a tilt at a second World Cup victory in rugby union.
"We knew he was coming back in 2015, we were just hoping we could get him back a year early," Rennie told Radio New Zealand.
"The offer was pretty attractive but in the end I think the loyalty to (Roosters chairman) Nick Politis who has given him a lot of support over the years when he went to the Roosters in the first place played a big part.
"I think this way he gets to have one more season of league before he re-commits to footy on this side of the ditch."
The NZRU earlier pledged to keep the "lines of communication" open with Williams, who under the guidance of manager Khoder Nasser has steered clear of signing long-term agreements since he controversially walked out on a five-year contract with the NRL's Canterbury Bulldogs in 2008.
The Roosters said Williams had "pledged his commitment" for 2014, but was yet to put pen to paper.
"The club anticipates that Williams will officially sign in mid-February 2014 for the remainder of the season on the same terms and conditions as his 2013 contract," the Roosters said on their website (www.roosters.com.au).
With up to four months to elapse until that tentative milestone, the statement is unlikely to prevent conjecture over Williams' intentions from dogging the club throughout the off-season.
Williams, himself, can expect his every movement to be scrutinised and dissected by suspicious media on both sides of the Tasman Sea.
The bulky dual code international's decision to walk away from the Bulldogs to play rugby union in France in 2008 was slammed as treachery in Australia's rugby league community.
Williams returned to New Zealand in 2010, playing one season with Super Rugby's Crusaders before crossing to the Chiefs.
Despite speaking of finding his "home" at the Chiefs in 2012, Williams returned to Australia's NRL this year on the strength of a handshake agreement with Politis.
The NZRU were already working on a deal to get him back to rugby union, added Rennie, who defended Williams' intentions.
"I don't think he has changed his mind at any stage," he said. "That's the reason why Khoder and him don't make a commitment because they don't like going back on their word. If they say they're coming, they're coming."
Williams created a storm earlier this week when he ruled himself out of New Zealand's title defence of the rugby league World Cup starting this month, then backflipped on his decision hours after the Kiwis squad was released.
He was controversially included in the squad at the expense of a younger player, a polarising move slammed by some New Zealand pundits.
A highly coveted player in both codes, Williams' retention by the Roosters would nonetheless be a big boost for the embattled NRL, which was rocked by a major anti-doping probe this season and other scandals involving player misbehaviour and referee blunders.
"It's fantastic to have Sonny stay for another season," NRL chief executive Dave Smith said on his Twitter feed.
"Great for the game, the fans and the Roosters. Fans can look forward to the World Cup."