Bell injured his knee in the second quarter in Sunday's 16-10 loss to the Bengals.
Bell caught a pass from quarterback Ben Roethlisberger and his knee twisted awkwardly underneath him when tackled by Bengals linebacker Vontaze Burfict. Bell did not put any weight on the leg as he was helped to a cart and taken to the locker room.
In six games this season, Bell totaled 556 rushing yards on 113 carries with three touchdowns, along with 24 receptions for 136 yards.
The Steelers' running game should be in capable hands with backup running back D'Angelo Williams, who broke a 55-yard run while finishing with 71 yards on nine carries Sunday.
The Steelers also signed free agent running back Isaiah Pead to take Bell's spot on the roster. Pead has played in 27 career games with one start, recording 78 rushing yards and 94 receiving yards. He was released by the St. Louis Rams on Sept. 29. Pead was originally drafted by the Rams in the second round (50th overall) of the 2012 NFL Draft.