(Reuters) - Boxing All Black Sonny Bill Williams said on Tuesday that his heavyweight clash with South African Francois Botha could be in doubt after he picked up an injury playing rugby in Japan.

Williams injured his shoulder playing for Japanese Top League club Panasonic Wild Knights at the weekend and was rushed to hospital for an MRI scan.

New Zealand's heavyweight boxing champion initially said on his Twitter page "results nothing serious, small tear so hopefully still be able to (box)."

But when Williams arrived in Sydney on Tuesday he said his pectoral muscle needed more tests, the results of which could see the November 24 fight with Botha in Brisbane called off.

"At the moment it is a little bit in doubt," he told reporters at Sydney airport before beginning his preparations to face the 44-year-old Botha.

"I just came back here straight away," added the 27-year-old. "I've got to get some scans and see how the pec is, see the extent of the injury."

Williams quit the world champion All Blacks and Waikato Chiefs for a lucrative 12-match deal in Japan worth an estimated $100,000 a game.

He smashed his shoulder into the turf after a tackle in Panasonic's 23-20 win over Kintetsu Liners on Saturday and left the ground with his arm in a sling.

The injury could potentially bring a swift end to the hulking centre's Japanese season, with Panasonic yet to see much of a return halfway through their investment.

Williams, who began his career in rugby league, had been expected to quit union to join National Rugby League (NRL) side the Sydney Roosters.

But interim NRL chief executive Shane Mattiske said earlier this week that Williams's return would only be cleared once the necessary documentation had been signed.

Williams said in June that he would honour a handshake agreement with Roosters supremo Nick Politis but to date, no contract has been put forward to the NRL.

"The Roosters understand that if they are to bring Sonny Bill to the game, they need to approach us about that," Mattiske said.

"They need to get to a point where they've got a contract they can bring to us for registration. At this stage we're waiting for that to happen."

(Reporting by Alastair Himmer in Tokyo; Editing by Patrick Johnston)

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