The second-year quarterback, getting the first starts of his career, was carted off the field and Denver listed his return as questionable.
Lynch entered the game, and his first regular-season pass was an incomplete attempt on third down.
However, the Memphis product, picked 26th overall in April, went on to lead the Broncos to a field goal on a drive that began with 1:14 left in the first half, giving Denver a 17-7 lead.
Siemian was 5 of 7 for 68 yards and a touchdown before getting hurt, and he was on the sidelines, with Lynch in the game, when the second half began.
Siemian was coming off of a game at Cincinnati in which he threw for four touchdowns, becoming the first quarterback in NFL history to throw for at least 300 yards with four or more touchdowns and no interceptions in his first career road start.