Caldwell suffered the injury in the first quarter of Sunday's 24-23 season-opening victory over Jacksonville. He was helped off the field, and Coach John Fox called the injury significant.
Caldwell, 32, signed with the Panthers on Sept. 2 after being released by the Chicago Bears. The Panthers had a need at outside linebacker since it was learned on Aug. 17 that Mark Fields would miss the entire 2003 season after being diagnosed with Hodgkin's disease.
Caldwell has started 49 of 150 games in 10 NFL seasons, and has 672 tackles, 14 1/2 sacks, and eight interceptions.
Last season, he started three of 16 games with the Bears, and had 69 tackles, three sacks, and a fumble recovery.
Caldwell enjoyed his best season with the Philadelphia Eagles in 2001, when he started all 16 games at weakside linebacker and recorded 140 tackles and three sacks.