The injury came late in the first half when Siemian was sacked and body-slammed to the turf by Buccaneers defensive tackle Clinton McDonald, landing hard on his left nonthrowing shoulder.
No penalty was called on the play.
Siemian did not return the game but after the game, he and Kubiak said he was available to return in case of an emergency.
First-round draft pick Paxton Lynch filled in gamely, leading the Broncos two two field goals and a touchdown. He completed 14 of 24 passes for 170 yards and one touchdown.
"Our plans moving forward are for Trevor to be there Sunday (against the Falcons) and be ready to go, but he's day to day," Kubiak told Denver Sports 760 on Monday.
Siemian said after the game would be "ready to play" on Sunday against Atlanta at Sports Authority Field at Mile High.