Super Rugby 2019: Sonny Bill Williams to undergo knee surgery, out six weeks

All Blacks midfielder Sonny Bill Williams will go under the knife on Friday for a knee injury that threatens to keep him side-lined for about six weeks and casts doubt over his selection for the Rugby World Cup later this year.

"It's a big blow," admits Blues coach Leon MacDonald. "I feel sorry for him.

"It's unfortunate for him - he landed on it awkwardly and caused some damage. Having a return-to-rugby plan would help with him.

"We'll miss him on the field obviously, but he's still contributing in the meeting rooms and where he can."

But MacDonald denied this could be the end for the 33-year-old, who has juggled careers across three contract sports since he was a teenager.

Williams has been a key contributor to two successful World Cup campaigns for the All Blacks and loomed as a leading candidate for the three-peat quest in Japan.

But he has been plagued by injury over the past couple of seasons and this is another sign that the body may be breaking down.

"Whatever he decides to do and puts his mind to, he seems to make happen," said MacDonald. "If he still wants to keep playing later on, he's the sort of guy that you never write off.

"He's a pretty determined guy and this year, he really wants to make the World Cup squad, so he's pretty driven to make sure he recovers well for this injury and gets back on the field."

But Williams has had knee problems previously.

"They're pretty cautious around how they're working with him," says MacDonald. "He's got some good advice and I'm sure whatever they decided, they will have long term in mind as well."

Williams joins a growing All Blacks injury list that currently includes halfback Aaron Smith, loose forwards Sam Cane, Liam Squire and Luke Whitelock, hooker Codie Taylor, prop Owen Franks and wing Waisake Naholo.

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