The Lions' defensive back was carted off the practice field Monday with a right leg injury after he was hurt during a team drill in which there seemed to be light contact.
After an MRI, a source told ESPN's Field Yates that Gardner-Johnson has no structural damage on the knee. Campbell confirmed that his status is day-to-day.
"It appears to be he's going to be OK. It doesn't appear to be anything serious," Campbell said Tuesday. "We'll see. The images look pretty good. He may be out a day or two, but yeah, seems to be OK."
Gardner-Johnson was in attendance during Tuesday's practice, but didn't participate in any drills, as he watched from the sidelines sporting a long black sleeve on his right leg.
"I'll be better," he told a group of reporters after practice in passing.
Lions cornerback Cameron Sutton called it a sigh of relief to learn that Gardner-Johnson's injury wasn't as bad as it appeared initially.
"Anytime you have any of your brothers go down, you have to take a moment to just put healing hands and prayers over them," Sutton said. "That's something that you never want to see and obviously, he's in great spirits, his energy is still high even coming off of that, and we'll just be excited to get him back when we can when he's healthy."
Gardner-Johnson couldn't put pressure on his leg as trainers helped him off the ground on Monday. But Campbell said he didn't overreact until the medical staff was able to determine the severity of his injury.
"First of all, any player that you see goes down, it affects you because you don't want that for any of your players, I don't care who it is. And I don't care if they're free agents, rookie free agents, high-priced guys, it doesn't matter," Campbell said. "You don't want that for anybody, but obviously, I've been around this long enough.
"Everybody reacts different to injuries, every player, and you just don't know and so, until you get the information, I don't let my mind go there and my next thought was, 'Well, [cornerback] Will [Harris is] in, let's go.' But obviously, to hear the news now, you feel a lot better about it."