Quarterback Jimmy Garoppolo will not play during the San Francisco 49ers' preseason opener on Saturday against the Dallas Cowboys at Levi's Stadium. That much has been known since July 26. Head coach Kyle Shanahan hopes to get Garoppolo into the team's second and third preseason games, both of which are matchups on the road.

Nick Mullens and C.J. Beathard are competing for the backup quarterback job. With Garoppolo sitting out the game against Dallas, Saturday is an excellent opportunity for one of the young third-year quarterbacks to prove himself.

Shanahan said after Thursday's practice that he had not yet determined which quarterback would start against Dallas. While the decision was probably made on Friday, Saturday's starter wasn't nearly officially known until Mullens took warm-up snaps with the first-team offense just moments before kickoff.

Sure enough, it was Mullens joining the starters (those who were not on the 28-player list of sidelined Niners) on the field once the Cowboys' first drive was complete. He is also a team captain for the game. Beathard will likely come in after him.

Below are the other 49ers who started the game.

RB: Raheem Mostert
WR: Dante Pettis, Richie James
TE: Ross Dwelley, Levine Toilolo
OL (LT to RT): Shon Coleman, Daniel Brunskill, Wesley Johnson, Najee Toran, Justin Skule

The 49ers signed Mullens as an undrafted free agent out of Southern Miss in 2017. He completed 176-of-274 attempts (64.2 percent) for 2,277 yards with 13 touchdowns and 10 interceptions through eight starts with the 49ers in 2018. Mullens replaced an injured Beathard during a Week 9 matchup against the Oakland Raiders and never relinquished the starting role.

Beathard was a third-round selection out of Iowa in 2017. He completed 102-of-169 attempts (60.4 percent) for 1,252 yards with eight touchdowns and seven interceptions through six games and five starts with the 49ers in 2018.