Siemian was driven to the turf on a sack by Tampa Bay defensive tackle Clinton McDonald. The team announced that Siemian suffered an injury to his left shoulder and his return is questionable.
Lynch, the team's first-round pick out of Memphis, stepped in to make his NFL debut. On the next drive, he completed passes of 16 and 18 yards to set up a 38-yard field goal by Brandon McManus.
Earlier, Buccaneers Pro Bowl defensive tackle Gerald McCoy injured his left calf and left the game.
McCoy came off the field slowly and was evaluated by trainers on the sideline, then walked gingerly to the locker room. The Bucs say his return is doubtful.
The Bucs have limited depth at defensive tackle, but backup Akeem Spence, filling in for McCoy, got a sack on the next possession.
The Broncos lead the Bucs 17-7 at halftime thanks to two Aqib Talib interceptions that led to short touchdown drives.