Bell, who was hurt in the second quarter of the Steelers' 16-10 loss to Cincinnati, has a severe MCL injury, ESPN NFL Insider Adam Schefter reports.
In 5½ games, Bell rushed for 556 yards and three touchdowns, along with 24 catches for 136 yards.
Free agent Isaiah Pead takes Bell's spot on the roster. The former second-round pick has 78 career rushing yards and 94 receiving yards in 27 games with the Rams. Bell will be a free agent after the 2016 season.
Assuming Bell makes a smooth recovery, the Steelers could still give him a contract extension leading up to next year. If the Steelers reach the postseason and Bell recovers by the end of the regular season, he would not be eligible to play in the playoffs.