Stafford suffered a subluxation of his right knee in which his knee cap popped out of its normal position but popped back in, a Lions source told ESPN Senior NFL Analyst Chris Mortensen. A series of X-rays and MRIs will determine if Stafford damaged any cartilage around the knee cap, but sources said such an injury is not normally a season-ender. He will undergo more tests in Detroit.
One medical source even suggested that Stafford could play next week with a brace, but that will be determined by the pain and stability of the knee. He also could miss a week or longer, pending more comprehensive tests.
The Lions had a third down at the Bears 6 when Stafford twisted his right knee as he was sacked for an 11-yard loss by Adewale Ogunleye.
That forced Detroit to kick a field goal, which cut Chicago's lead to 34-24 with 8:15 left. Daunte Culpepper replaced Stafford on the Lions' next possession, and Stafford got the knee wrapped on the sideline.
"If I was feeling a little bit better they would've let me back in," said Stafford, who threw for 296 yards.
Johnson, who had 133 yards receiving, also left the game late with a leg injury, although the nature of the problem wasn't clear.
"I tried loosening my legs up, they got a little tight," he said. "I'll be all right, though."
Linebacker Ernie Sims also aggravated a right shoulder injury and left the game late in the third quarter.
Information from The Associated Press and ESPN Senior NFL Analyst Chris Mortensen was used in this report.