Losing Bush is another blow for the struggling Bears. They've dropped five of six after a 7-1 start and are coming off a loss to Green Bay that clinched the NFC North for the Packers.
Chicago needs help to make the playoffs after leading the division for much of the season, and losing Bush does not make things any easier.
Bush, who was signed to a four-year deal to give the Bears depth in the backfield, has been suffering from cartilage damage in his ribs. He dressed for Sunday's game against Green Bay but did not play. Bush has played in 13 games this season, gaining 411 yards and five touchdowns on 114 carries.
Bell has played in 25 games -- including three starts -- with the Bears and New York Jets in the past four seasons.
Information from The Associated Press was used in this report.