"Just spoke to the doctor operation went well, Thanks for the support I've truly been blessed," the 27-year-old tweeted to his followers.
Williams injured his shoulder and chest while playing for Japanese Top League club Panasonic Wild Knights at the weekend.
New Zealand's heavyweight boxing champion flew to Sydney on Tuesday to prepare for his November 24 bout against 44-year-old Francois Botha in Brisbane.
However, Williams now faces up to two months out, which would leave little option but to postpone his fight with Botha, while his return to Japanese rugby could also be in doubt.
The hulking centre left the world champion All Blacks and Waikato Chiefs for a lucrative 12-match deal in Japan worth an estimated $100,000 a game.
Williams smashed his shoulder into the turf after a flying tackle in Panasonic's 23-20 win over Kintetsu Liners on Saturday and was rushed to hospital for an MRI scan.
The injury could spell a premature end to his Japanese season, with Panasonic having seen little return halfway through their investment.
Williams is expected to quit union and return to rugby league with the Sydney Roosters next year.