LAKE FOREST, Ill. -- Chicago Bears running back Matt Forte has been ruled out of Sunday's game against the St. Louis Rams because of an ankle injury, coach Lovie Smith said.

"Matt Forte is making a lot of progress but he won't be ready to this week, so he is out for this game," Smith said.

Forte, who missed his fourth straight practice Friday, suffered a right ankle sprain in the third quarter of the Bears' loss to the Green Bay Packers on Sept. 13.

Michael Bush will start in place of Forte. Signed to a four-year, $14 million contract in the offseason, Bush had success last season with the Oakland Raiders replacing another injured star running back, Darren McFadden. Bush started nine games last season and rushed for a career-high 977 yards.

"Hopefully, I can get people to get that knock off me about not being an every-down back," Bush said. "But I can't please everybody. It's a business, it is what it is. If you don't like me, so what. I'm going to keep on playing hard.

"I still think I need to prove to the coaches each week that I get a chance. The more I get to play, the more I get to show them, and the more (I hope) they'll trust in me, believe in me and give me more opportunities to more things. But for the most part, I know they've got my back, regardless."

Bush has rushed for 96 yards on 26 carries with two touchdowns this season.

"Michael has started a lot in the league and has been in this role before," Smith said Friday. "There are not many Matt Fortes out there but our backup plan is pretty good, and Mike's going to step in there and do a great job."

Armando Allen and Kahlil Bell are the other two running backs on the Bears' 53-man roster and could figure into the game plan on Sunday.

Defensive end Julius Peppers is listed as probable on the Bears' injury report with a foot injury, and he has had full participation in practice the past two days.

For the Rams, running back Steven Jackson (groin) is questionable and did not practice on Friday. Also questionable are offensive tackle Wayne Hunter (ankle) and fullback Brit Miller (thigh).

Defensive tackle Michael Brockers (ankle) is doubtful, and defensive tackle Matt

Conrath (knee), safety Matt

Daniels (hamstring) and offensive tackle Rodger

Saffold (knee) are out.