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.
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.
"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."
The Associated Press contributed to this report.