Fairley, selected 13th overall in April's draft, left practice Monday with a left foot injury. X-rays later that day were normal, according to the Detroit Free Press.
Fairley wore a walking boot Tuesday, and the Lions' medical staff might be exploring a possible stress fracture, the News reported.
"We are still doing some tests," coach Jim Schwartz said. "We are having a lot of different people looking at it. He's down for now."
Schwartz would not say if he believes the injury is long-term or short-term in nature.
The coach described how the injury occurred on Monday.
"Nick sort of tweaked his foot during pass-rush, or individual (drills), one of those things," Schwartz said. "They sent him in to get it X-rayed, just to make sure there was nothing, no broken bone in there, because he wanted to keep on practicing. So we sent him in, and that was negative.
After the injury occurred, Fairley didn't seem concerned.
"Probably tweaked it a little bit," Fairley said. "It's OK."