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Starting quarterback Jason Campbell sustained a potential season-ending knee injury in the second quarter of the Washington Redskins' game against the Chicago Bears on Thursday night, as the third-year veteran dislocated his left patella.

Team sources and a source close to Campbell said late Thursday that an MRI examination will probably be performed Friday, and the team should know more about the injury after the test. But the Redskins are bracing to finish the season without Campbell, who had started the first 13 games of the year.

Campbell, 25, was injured with less than three minutes to play in the first half on a pass completion to tailback Ladell Betts for a 7-yard gain. As he threw, Campbell was hit by Chicago defensive end Mark Anderson and Campbell's knee buckled awkwardly under him.

As he lay on the ground in obvious pain, Campbell tossed his helmet. His left leg was immobilized in an air cast, and he was eventually carted off the field.

Earlier in the quarter, Campbell briefly left the game when he was hit on his right elbow. He returned after throwing a few passes on the sideline. After the knee injury, Campbell was replaced by journeyman Todd Collins, who recorded his first pass attempt since 2004.

If Campbell is sidelined, as anticipated, Collins, a 13-year veteran who has not started a regular-season game since 1997, will probably move into the lineup. The other quarterback on the roster is 15-year veteran Mark Brunell.

In his 13 games this season, Campbell completed 240-of-401 passes for 2,600 yards, with 12 touchdown passes and 11 interceptions, for a passer rating of 77.5. In his first full season as the Washington starter, Campbell demonstrated flashes of brilliance, and he recorded the the first two 300-yard passing games of his career. But those moments were offset at times by streaks of inconsistency and a tendency for carelessness with the ball.

In addition to his 11 interceptions, Campbell lost nine fumbles, including at least one in each of his last five starts.

The latter of the team's two first-round selections in the 2005 draft, Campbell did not play a single snap as a rookie. The former Auburn star supplanted Brunell as the starter for the final seven games of the 2006 season and then went to camp this summer as the starter.

Len Pasquarelli is a senior NFL writer for ESPN.com.