The worst fears of South Sydney and New Zealand captain Roy Asotasi were realised on Saturday when he was ruled out of the coming World Cup with a torn pectoral muscle.

The 26-year-old prop suffered the damage in the closing moments of on Friday night's 22-20 loss to the Sydney Roosters.

After the game, Asotasi was pessimistic about his World Cup prospects and the initial evaluation by the Rabbitohs medical staff was proven accurate by Saturday's scan.

"The MRI showed he has a pectoral tear and he will be operated on, on Monday afternoon and he will be out for between 18 and 22 weeks," Souths director of football operations Shane Richardson told AAP.

"From our point of view, he will be back for the start of next season which is important for us, but it's sad for him and the New Zealand Test side."

Richardson refused to comment on the tackle which Souths coach Jason Taylor wanted to check.

There was a suggestion within the Rabbitohs camp that the injury could have been caused by a "chicken wing" tackle.

Taylor said he was testing different combinations against the top teams over the closing rounds of the competition.

His tinkering with his team included moving regular five-eighth Craig Wing to hooker and lock John Sutton from lock to five-eighth.

He opted to put regular starting hooker Isaac Luke on the bench.

"We just wanted to make some adjustments there, we are using these games to work on our combinations of John (Sutton) and Craig (Wing) and (halfback) Chris (Sandow) and Isaac Luke," Taylor said.

"I started the game with Wingy at hooker which we haven't done since he's been at the club and I thought it worked really well.

"We brought Isaac off the bench a bit fresher, it was just a bit of my having a look at the best combination going forward."

Taylor viewed the Rabbitohs' challenging late season schedule, which pitted them against current top six teams in each of the last five rounds, as a good opportunity to assess different combinations.

"We are using these games to learn a lot. Over the last few weeks we knew we weren't going to be in the semi-finals," Taylor said.

"But we're playing the best teams in the competition and we're not turning up and just going through the motions, because we're going out there and really ripping into them."

While Souths appeared unlucky not to get a 40-20 call in their favour in the closing seconds of Friday night's game, Taylor wasn't sure if video referee Bill Harrigan made the right call as he didn't get a clear view of the incident.

However, Taylor thought the Roosters were lucky to be awarded their first two tries and especially their first, where he thought there was a clear knock on.

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