Coach Chan Gailey declined to say which foot Mitchell injured, or to release details except to say he was hurt in practice a day earlier and is "doubtful" to play. Mitchell was having tests to determine what Gailey called a "short-term and long-term" prognosis, an indication the injury could be severe.
Mitchell, a seven-year NFL veteran, is a backup to Andra Davis, who's taken over the defensive play-calling duties this season.
Mitchell is coming back after he missed the final 11 games last season with a right knee injury.
Gailey was optimistic starting safety Jairus Byrd will play after missing nearly a month after having groin surgery. "He was out there and moved around really well today at practice. It's an improvement over yesterday," Gailey said.
Byrd earned a Pro Bowl selection as a rookie last year after finishing tied for the NFL lead with nine interceptions.