But before arriving in Europe the two teams will go head-to-head in the final Bledisloe Cup match of the year, staged in Hong Kong.
There has been plenty of hype in the build-up around New Zealand's latest rugby league convert, but Cooper is adamant that Williams is worth every column inch.
"He is a giant," Cooper told the Sun Herald.
"He can defend hard, he can put a massive hit on, he can break tackles and offload with one hand. It's crazy. He's big, man. He's got some massive shoulders.
"He's built like a bodybuilder, and prancing and running around, it's not going to be the most exciting thing trying to tackle him."
Williams returned to New Zealand to take up a contract with Canterbury in the summer after two years at Toulon.
The former Canterbury Bulldogs player and professional boxer was a sensation in France's Top 14 last year and Cooper believes the 25-year-old centre will only get better in an All Black shirt.
"I'm where I'm at now because I was lucky enough to go on the spring tour last year, and I think Sonny will benefit just as much," continued Cooper.
"Coming into a World Cup, you want to play against the best nations - the Wallabies, the Springboks and then a spring tour of the power nations.
"It's going to be great for him, and I know he'll really enjoy it and turn into an even better player than what he is now.
"It's going to be scary.
"And him running off Dan Carter, that's not something I like thinking about. But for him, being part of that set-up, for his personal development it's going to work wonders for Sonny."