Chicago Bears strong safety Mike Brown is out indefinitely after undergoing surgery to repair ligament damage in his right foot Wednesday, a major hit for the NFL's second-ranked defence.

Brown limped off the field and was taken from the sideline on a cart during the fourth quarter of Monday's 24-23 comeback victory at Arizona after his foot got twisted on a running play.

``Severe blow,'' coach Lovie Smith said. ``I think we all know what Mike means to our football team. He's a special player. He was having an outstanding game. ... That's a big blow, but injuries do happen. Mike has been in this situation before.''

Smith wasn't sure if Brown, a Pro Bowl pick last season, would be placed on injured reserve.

Brown played a key role as the Bears rallied from a 20-point deficit against the Cardinals.

Brown's injury leaves the Bears thin at safety. Backup Chris Harris has missed three of the past four games with a pulled quadriceps and a sprained ankle. Brandon McGowan was suspended by the team for a week for violating a team rule, but is expected to be back Monday.

Brown's injury could mean more playing time for Todd Johnson, who replaced him on Monday, and Cameron Worrell.

``The encouraging thing is we have guys who can step in and make plays for us,'' cornerback Nathan Vasher said.