Entering Soldier Field Sunday afternoon you have two teams notorious for vicious defenses but both missing the leaders of those defensive units.

While it may seem like a similar situation, the reality is entirely different.

The Bears' 31-year old middle linebacker, six-time pro bowler and former NFL Defensive Player of the Year Brian Urlacher is out for the season with a dislocated wrist.

The Steelers' 28-year old safety, five-time Pro Bowler Troy Polamalu is out anywhere from three to six weeks with a sprain medial collateral ligament.

Obviously the impact on each team is different in the long run as Urlacher's injury will keep him out for the remainder of the season while Polamalu has a chance of returning by the fifth week of the season.

However, in the short term the injuries' impacts are also different. Undoubtedly both players are in the one position on defense their respective teams can least afford to lose. Urlacher is the emotional leader of the defense and the quarterback of the unit, calling out plays and blitzes.

On the Steelers linebacker and defensive co-captain James Farrior handles those duties. Another linebacker and co-captain James Harrison is the reigning NFL Defensive Player of the Year.

To fill in for Urlacher the Bears are moving Hunter Hillenmeyer to middle linebacker after toying with the possibility of bringing in veteran Derrick Brooks. The Steelers are moving Tyrone Carter, a nine-year veteran, into the role he has played numerous occasions.

Each team has also had experience playing without their respective defensive leaders. Urlacher was missed for seven games in 2004 with calf and hamstring injuries. Polamalu was out for three games with the same injury in November and December of 2007.

Both teams played completely different.

The Bears did not win a single game during Urlacher's seven-game absence. The defensive unit allowed 26 points per game while going 0-7. Compare that to the 5-4 record and 16.6 point per game average the defense maintained with Urlacher in the lineup. Oh yeah, and about his replacement in 2004, it was Hillenmeyer playing during that 0-7 stretch. Considering the Bears blew a fourth quarter lead to the Packers Sunday night without Urlacher in the lineup, Chicago has not won a game without him in a decade.

With Polamalu out in 2007 the Steelers went 2-1 and the defense shut out one opponent and limited opponents to 14.6 points per game.

While the Bears were seriously considering bringing in a veteran in Derrick Brooks the Steelers never flinched and simply moved a proven veteran and Super Bowl winner Tyrone Carter over to the position. If something else were to happen, Ryan Mundy, a second-year player out of West Virginia, who the Steelers are very impressed with, will be waiting in the wings.

Two defensive leaders and two teams learning to play without them, both teams appear to be in the same predicament but the situations could not be further for reality.

Las Vegas has made the Steelers a three-point favorite for NFL odds this Sunday against the Bears. It is the first time since early November of last season that the Bears are underdogs at Soldier Field and considering the defensive situation on both teams, it's with good reason.