Viking quarterback Tarvaris Jackson is preparing to play Saturday against Pittsburgh despite a sprained MCL in his right knee.  Jackson did light individual workouts with the Vikings yesterday while wearing a brace on the knee.  The Vikings host Pittsburgh Saturday night at 7pm, pregame on AM 1390-the Fan at 6pm.  Minnesota had to place defense tackle Kendrick Allen on injured reserve with a wrist injury.  Allen is done for the year.  The Vikings are thin on the defense line with injuries to Jayme Mitchell, Brian Robison and now Allen.