The Minnesota Vikings are missing a couple of key backups to injuries.

Coach Mike Zimmer says Wednesday that defensive tackle Shamar Stephen had arthroscopic knee surgery and offensive tackle Carter Bykowski had shoulder surgery.

Stephen was in the rotation as a rookie last year and started three games when Sharrif Floyd was hurt. Zimmer said he expects Stephen to return by the regular-season opener Sept. 14. Stephen has been bothered by knee trouble throughout training camp.

Safety Antone Exum has also been out of practice this week because of a hamstring problem. He’s been challenging Robert Blanton for the starting strong safety spot.