Ty Detmer would start if McMahon can't play against the Bears (11-3). Detmer has not taken a snap since he started the season's third game against St. Louis, which followed his seven-interception performance the previous week at Cleveland.
Lamont Warren will start for Detroit (1-13) if Stewart can't play.
Linebacker Barrett Green also missed practice with a back injury, and was downgraded from questionable to doubtful.
Fullback Cory Schlesinger and wide receiver-kick returner Desmond Howard practiced Thursday after missing the Dec. 26 workout because of illness.