Detroit Lions rookie quarterback Matthew Stafford suffered what the team's medical staff believes could be a dislocated right kneecap during Sunday's 48-24 road loss to the Chicago Bears, a league source told NFL Network's Jason La Canfora.
Veteran backup Daunte Culpepper replaced Stafford at quarterback on the Lions' next possession, and the rookie had his knee wrapped on the sideline. But Stafford, the No. 1 overall pick in the April draft, didn't seem overly concerned about it.
"If I was feeling a little bit better, they would've let me back in," said Stafford, who threw for 296 yards and one touchdown with one interception in his fourth career NFL start.
The extent of Stafford's injury -- and how much time he could miss, if any -- will be known after further tests are conducted Monday.
"He wasn't able to get it warmed back up again and we didn't want to put him back in the game," Lions coach Jim Schwartz said. "If he was healthy enough, he would have gone back into the game."
The Lions declined to comment when asked if Stafford had suffered a dislocated kneecap. A team spokesperson e-mailed a response that Stafford "was not physically able to finish the game" and that he will have a further update Monday.
Lions wide receiver Calvin Johnson also left in the fourth quarter with a leg injury, although the exact nature of the problem wasn't clear.
"I tried loosening my legs up, they got a little tight," said Johnson, who had a team-high eight catches for 133 yards against the Bears. "I'll be all right, though."
Lions linebacker Ernie Sims also aggravated a right shoulder injury and left the game late in the third quarter.
The Associated Press contributed to this report.
