Blues centre Sonny Bill Williams ruled out for six weeks

Sonny Bill Williams runs the ball during the Blues' win over the Stormers in Auckland, March 31, 2019 MICHAEL BRADLEY/AFP/Getty Images

Blues centre Sonny Bill Williams is set to miss at least six weeks of action after he undergoes surgery on his problematic knee tomorrow.

Williams hurt his right knee during the Blue's win against the Stormers at Eden Park and was forced out of their win over the Waratahs last weekend.

It was decided Williams would need arthroscopy surgery to clean up a cartilage issue that has been dogging the star midfielder for some time, sidelining him until at least the end of May.

This puts a serious dent in his All Blacks World Cup ambitions. Williams was originally named in the All Blacks 41-man squad for foundation day in Wellington on Monday, but is certain to miss it. No other player has been called up as a replacement.

"It's a big blow," Blues coach Leon MacDonald said on Thursday. "I feel sorry for him. It's unfortunate, he's obviously landed on it awkwardly and has caused some damage there.

"For him to have a return-to-rugby plan will help. We'll miss him on the rugby field, obviously, but he's still contributing in the meeting rooms and where he can."

With just five months until the All Blacks opening match against South Africa at the World Cup, the clock is ticking for Williams who's struggled with injury previously, but MacDonald remains cautiously optimistic that the 33-year-old will be available.

"Whatever he puts his mind to, he makes happen," he said. "If he still wants to play later on he's the sort of guy you'd never write off. This year he really wants to make the World Cup squad and be a part of that obviously, so he's driven to recover well from this injury and get back on the field."

Williams has previously struggled with an ankle, wrist and shoulder problem over the past few years.