NRL-bound Sonny Bill Williams is confident that he will be fit to fight South African Francois Botha in a heavyweight boxing match next month after suffering a shoulder injury.

On a short playing stint with Panasonic Wild Knights in Japan's rugby union Top League, All Blacks centre Williams was taken to hospital after injuring his right shoulder in a match against Kintetsu Liners in Ota, near Tokyo, on Saturday.

"Hurt today .. results back nothing serious, small tear so hopefully still be able to (fight Botha)," Williams wrote on Twitter hours later.

His first tweet after sustaining the injury said: "Going in for MRI hopefully just a small tear?!!"

It was accompanied by a picture showing his right arm in a sling and his left hand clenched as if he was ready to fight.

Williams is expected to return to rugby league with the Sydney Roosters in the 2013 NRL season.

A part-time boxer, he is the New Zealand heavyweight champion and is scheduled to take on veteran Botha, 44, a four-time world title challenger, in Brisbane on November 24 in his sixth pro bout.

Williams, who is on a contract to play 12 matches for Panasonic reportedly for $1.2 million, scored a first-minute try, his second since joining the team, in the Top League eighth round.

But he crunched his shoulder attempting a tackle on former Western Force centre Gene Fairbanks 10 minutes into the match.

He left the field and Panasonic went on to beat Kintetsu 23-20 for their fifth win against three losses.

Panasonic stood fifth in the league table while defending champions Suntory Sungoliath are top.

The Top League has now gone into a five-week break with its final set for late January.