Siemian, who has yet to throw a regular-season pass in the NFL, will start against the Carolina Panthers on Sept. 8 to kick off the NFL's 2016 regular season.
"The thing about replacing Peyton -- nobody is going to replace Peyton," Siemian said last week. "Those shoes are a little too big to fill. For me, I'm not reading too much into anything and trying to be the best teammate and leader I can be."
The 24-year-old Siemian won the battle with Mark Sanchez, who was traded in March by the Philadelphia Eagles to the Broncos for a conditional 2017 draft pick.
Siemian solidified his hold on the starting job in Saturday night's third preseason game when the Broncos beat the Los Angeles Rams 17-9, completing 10 of 17 passes for 122 yards with a touchdown and an interception that came off a deflection.
"I'm impressed -- I think he's very calm,'' Kubiak said of Siemian after the game. "I can tell by the way he handles the team in the huddle, he's got control of what's going on. Gets a bad break on the 'go' ball and comes right back and goes down the field. I think what I see is a guy getting better."
Sanchez, who didn't play in the third game, is slated to count $4.5 million against the salary cap if he makes the roster. The Broncos would not have to surrender the pick to the Eagles if he is not on the roster to open the season.
The Broncos moved up five spots in the first round of the 2016 draft to select Lynch, who was 6-of-13 passing for 57 yards and led a touchdown drive Saturday night.