Tokyo - New Zealand All Blacks centre Sonny Bill Williams was taken to hospital for an MRI scan after hurting his shoulder playing for Japan's Panasonic Wild Knights on Saturday.
The New Zealand heavyweight boxing champion said on his Twitter page: "Going in for an MRI hopefully just a small muscle tear" after Panasonic's 23-20 win over Kintetsu Liners.
Williams, who scored a first-minute try, was taken to hospital after the match with his right arm in a sling.
If serious the injury could put his fight against South African Francois Botha in Brisbane next month at risk and even bring a swift end to his Japanese season.
Williams left the world champion All Blacks and Waikato Chiefs for a 12-match deal in Japan worth an estimated $100,000 a game.
Panasonic have yet to see a bumper return on their investment, with the Wild Knights fifth in Japan's Top League standings, 14 points behind leaders Suntory Sungoliath.
Last year's runners-up Panasonic have yet to click and an injury would pile more pressure on Williams, who has already admitted to feeling the weight of his huge price tag.
Defending champions Suntory continue to make it look easy, however, thrashing Fukuoka Sanix Blues 42-19 to make it eight wins from eight.
Record holder Hirotoki Onozawa's 100th Top League try ensured that Suntory go into the month-long winter break four points clear at the top with 39 points from a possible 40.