Lock went to the locker room in the first quarter and never returned. His right arm was in a sling after the game.
The second-year signal-caller appeared to suffer the injury while being sacked by Steelers pass-rusher Bud Dupree. Lock fumbled on the play.
Jeff Driskel replaced Lock at quarterback, tossing a pair of touchdowns and an interception. The Broncos dropped their second straight decision to open the season.