Williams' manager George Peterson told the Associated Press that Williams has been listed in serious but stable condition on Monday after being involved in a motorcycle crash in Atlanta over the weekend.
According to reports, the Sunday-morning crash took place in Marietta after Williams, 30, attempted to avoid another car trying to negotiate a curve.
The boxer's motorcycle eventually went up an embankment as Williams was thrown from the bike, severing his spinal cord after falling on his back and head.
The former two-division world champion was scheduled to fight Saul Canelo Alvarez in a pay-per-view headliner on Sept. 15 in Las Vegas.
Williams (41-2, 27 knockouts), is a former two-time welterweight titleholder and former interim junior middleweight titlist.
He also won a majority decision over Sergio Martinez, who went on to win the middleweight championship, and he claims victories over Winky Wright and Antonio Margarito.
Williams made his boxing debut in 2000.
Paul Williams Highlights