Williams hobbled off in the final minute of Carolina's win over San Francisco last Sunday.
The Panthers' hope by midweek was to be able to make a game-time decision on playing Williams, but coach John Fox saw so little this week that thinks it is doubtful Williams will play.
Williams is Carolina's top rusher with 361 yards on 87 carries. But he's far behind last year's pace, when he and Jonathan Stewart each rushed for more than 1,100 yards.
Stewart, who has just 148 yards on 50 carries, and Mike Goodson are in line to get most of the carries against the Rams if Williams is sidelined.
Reserve receiver Devin Thomas also missed practice Friday and is doubtful with a groin injury.
Right tackle Jeff Otah (knee) will miss his seventh straight game.
Information from The Associated Press and ESPN.com senior NFL writer John Clayton was used in this report.