DENVER — Backups never know when it’s going to happen. The last time EJ Manuel stepped into a regular-season game was New Year’s Day and he was wearing a Buffalo Bills uniform, as he had for the first four seasons of his career. On Sunday, he got his first regular-season action with the Raiders, forced into the game late in the third quarter, when Derek Carr left with a back injury. “Of course, we don’t want anything to happen to Derek,” Manuel said, “but I got some snaps.