Bills coach Chan Gailey declined to say which foot Mitchell injured or release details except to say the linebacker was hurt during practice one 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 has taken over the defensive play-calling duties this season.
Mitchell is coming back after he missed the final 11 games last season because of a right knee injury.
The Bills already are expected to be without outside linebacker Reggie Torbor, who's recovering from a chest injury. Converted defensive end Chris Ellis is expected to start in place of Torbor.
"He was out there and moved around really well today at practice," Gailey said. "It's an improvement over yesterday."
Byrd finished tied for the NFL lead with nine interceptions last season.
Copyright 2010 by The Associated Press