Williams suffered a high ankle sprain in the first half of a Sept. 28 loss to Baltimore. He has missed the past two games against Chicago and Cincinnati after missing Weeks 2 and 3 with a hamstring injury.
Until Monday, Rivera had not been specific about how long the team's all-time leading rusher would be out. Despite playing less than six quarters, Williams was the team's leading rusher this season with 106 yards until quarterback Cam Newton rushed for 107 yards on 17 carries in the Panthers' 37-37 tie with the Bengals on Sunday. Newton now has 149 yards rushing. The next closest among healthy backs is Darrin Reaves with 76 yards.
Rivera said Jonathan Stewart, who has missed three straight games with a knee injury, could be back for Sunday's game at Green Bay. Stewart had 88 yards rushing in the first three games.
Newton's emergence as a runner for the first time since undergoing ankle surgery in March should help a Carolina team ranked 28th in rush offense.
"Now all of a sudden those defensive ends can't go crashing down, or those linebackers -- or what we call the hangovers -- can't start driving at the runner," Rivera said. "They've got to sit there and read. Now all of a sudden they're not getting good jumps to the ball carrier, so they're not quite sure who has the ball."
Meanwhile, Rivera said cornerback Josh Norman (concussion) will be re-evaluated by doctors on Tuesday and nickel cornerback Bené Benwikere (ankle) is doubtful for Green Bay.