Gardner-Johnson was carted off the practice field with the injury on Monday after he was hurt during a team drill in which there seemed to be light contact. He couldn't put pressure on his right leg as trainers helped him to his feet.
Gardner-Johnson's teammates were shaken up in the immediate aftermath of the injury.
"It's hard to see your brothers go down. You never want to see it, especially early in camp or really any days, regardless of the position or the player," cornerback Cameron Sutton told ESPN after practice. "It's kind of hard to detail. All you can do is send up prayers."
Quarterback Jared Goff and several teammates went to check on him. Gardner-Johnson was visibly emotional after the play, as he pointed near his right upper leg to trainers while lying on the practice field. After practice, members of the team were quick to send encouragement as they waited to learn the official status of the injury.
"When I went over there, I think he said somebody stepped on it, but I didn't see it. I just seen him go down and I seen him grabbing it. I went through that, too, where I grabbed mine," cornerback Jerry Jacobs said. "Not saying that I know what it is, but I'm just praying that it's not nothing like [an ACL]. To see a guy like that with great energy and a great attitude to change the room. S--- is so sad. I'm going to go in and check on him. But it's sad because he's a great guy."
The Lions brought him in as their top free agent for a major upgrade to the secondary. He agreed to a one-year deal worth up to $8 million in March.
Last season, with the Philadelphia Eagles, Gardner-Johnson finished tied for the most interceptions in the NFL (six) despite missing five games with a lacerated kidney. He became the only Eagles player since the 1970 merger to record interceptions in five consecutive games.
"You hate when people go down, especially without pads on -- it's camp. We haven't even played a game," linebacker James Houston told ESPN. "So, first, I'm praying for him, and I think the whole team is praying for him and we just hope he's going to be OK.
"You can't let things like that affect you. The show must go on," he continued. "Somebody else has to go in and fill in and we've got to be able to keep the beat going. Obviously, yeah, it sucks, because that's your homeboy, that's your teammate. I've known him for a very long time, so I definitely will be praying for him."