At his Tuesday press conference, Caldwell said he isn't sure whether Fairley is out for the season.
Fairley was knocked out of Sunday's game at London's Wembley Stadium after he collided with fellow defensive tackle Ndamukong Suh.
Caldwell was asked about Fairley's status during a radio appearance on Monday.
"[Doctors] have looked at him and obviously he's not going to be back quickly," Caldwell said during a radio interview with 97.1-FM on Monday night. "That's one of the things that we certainly know, but how long it's going to be, we're not quite certain."
The Lions seemed to be concerned that the knee injury was serious immediately after the game, according to The Detroit Free Press. It was later reported that the team hoped Fairley would only be out 4-5 weeks, but it remains possible that the defensive tackle will remain on the sidelines much longer.
The former Auburn star has battled injury and weight issues throughout his four-year career, and the team declined to pick up the fifth-year option on his rookie deal, making him a free agent in 2015. After he was benched during the preseason, Fairley's play has elevated during the regular season, helping the Lions become the NFL's No. 1 defense so far this year.
Fairley has recorded 14 tackles with one forced fumble and a sack this season.
The Lions are 6-2 entering their bye week after their 22-21 comeback win over the Falcons. Their next game is at home against the Dolphins on Nov. 9.
- Jack Jorgensen