Foster has a bruised knee, according to general manager Marty Hurney.
However, he missed all week leading up to the Detroit game and did not play in the game. Then he had all of last week off because of the bye week, but apparently that wasn't enough time off.
When the Panthers returned to practice on Monday to prepare for the Minnesota Vikings, Foster again sat out practice.
It's unclear if the injury is anyway related to the micro-fracture knee surgery he had that forced him to miss all of the 2002 season.
Foster has been a valuable change-of-pace back for the Panthers this season, averaging 74.6 all-purpose yards per game from scrimmage. The Panthers really struggled to run the ball against the Lions with Foster out of the lineup.
And while Carolina may be able to get by without him this week against Minnesota, they would certainly like to have him back for the following Sunday's crucial match-up with first place Tampa Bay.
Foster was unavailable for comment on Monday.
"It's all hard," Fox said. "Everything about this profession is hard. Whether it's on the field or off the field, it's a tough business and it takes some tough people. There's not too much about it that's easy."