Williams hobbled off the field in the final minute of Carolina's win over San Francisco last weekend and hasn't practiced since. The team has provided no update on the injury and Williams hasn't talked to reporters.
Saturday's decision means Jonathan Stewart will likely get the start against the Rams as the Panthers try to jump-start the league's 23rd-ranked rushing offense.
After Williams rushed for more than 1,100 yards last season, the 2009 Pro Bowl selection has been held to 361 yards on 87 carries. Stewart has 148 yards on 50 carries.
Mike Goodson will likely be the No. 2 back Sunday.