Paxton Lynch will get the first start of his NFL career Sunday when the rookie quarterback leads the Denver Broncos in their non-conference game against the Atlanta Falcons.

Multiple NFL sources, including several player agents, told KUSA that Trevor Siemian would not start against the Falcons because of his shoulder injury. However, sources indicated Siemian, the Broncos’ starting quarterback through their first four games, could be on track to start Thursday night against the San Diego Chargers at Qualcomm Stadium.

Siemian suffered a sprained AC joint to his left non-throwing shoulder last Sunday in the Broncos’ 27-7 win at Tampa Bay. Siemian left with the Broncos ahead 14-7.

Lynch completed 14-of-24 passes for 170 yards in his NFL debut. Now he gets his first start against a Falcons team that ranks No. 1 in the league in offense (478.8 yards per game) and scoring (38 points per game) but ranks 30th in total defense (419.3 yards per game) and 29th in points allowed (31).

It’s unclear whether Siemian or Austin Davis will serve as Lynch’s backup Sunday, but it figures that if Siemian is not healthy enough to start, he wouldn’t be deemed healthy enough to play.

Davis is a fifth-year quarterback whom the Broncos signed to a one-year, $1.25 million contract after the team released Mark Sanchez as part of their final cuts on Sept. 3.

Davis, 27, started eight games for the St. Louis Rams in 2014 and two for the Cleveland Browns last year.

The decision to start Lynch was hardly a surprise after watching Siemian throw in practice this week. The Broncos have posted a 4-0 record with Siemian as their starting quarterback — which is why Broncos head coach Gary Kubiak said Siemian would remain the starter against Atlanta if the second-year quarterback could sufficiently recover from his injury.

The large lump and deep bruises that were visible on Siemian’s left shoulder this week, though, were signs he needed more time to mend. Siemian was clearly favoring his left shoulder during the individual practice periods that are open to the media for viewing on Thursday and Friday. Siemian kept his left elbow tucked close to his body as he tossed the ball with his right arm.

He did take a few team reps on Friday, but the concern was not only whether Siemian was healthy enough to play against the Falcons but whether the shoulder was strong enough to potentially absorb more contact.

Siemian suffered the injury late in the second quarter against Tampa Bay as he was getting sacked by defensive tackle Clinton McDonald.

Lynch came in and showed he was ready. Now he gets his first chance to lead the Broncos from the start.

PHOTOS | Atlanta Falcons

Mike Klis reports for KUSA in Denver, a TEGNA property.