Gardner-Johnson was carted off the field Monday due to a non-contact injury, Tim Twentyman of the Lions' official site reports.
Adam Schefter of ESPN reports that Gardner-Johnson's injury was not caused by contact, a potentially disastrous development for Detroit's biggest free-agency addition on defense. The veteran cornerback had taken first-team reps at slot cornerback this offseason and also was expected to contribute at safety, so it will be a brutal blow for the Lions secondary if he ends up missing significant time.