Third-stringer C.J. Beathard will start at quarterback for the 49ers in Saturday’s game against the Arizona Cardinals following an injury to No. 2 QB Nick Mullens.

Niners coach Kyle Shanahan said Tuesday that Mullens, who injured his right elbow in the fourth quarter of Sunday’s 41-33 loss to the Dallas Cowboys, will miss the rest of the season and might require Tommy John surgery to repair the injury. More testing will be done on Mullens’ elbow before a final determination is made.

Beathard becomes the third QB to start this season for the 49ers. Jimmy Garoppolo has not played since aggravating a high ankle sprain in Week 8 vs. the Seattle Seahawks, an injury he originally sustained Week 2 against the New York Jets.

Josh Rosen, the 10th overall pick of the 2018 draft out of UCLA, has been signed off the Tampa Bay Buccaneers’ practice squad, according to an ESPN report, and would be Beathard’s backup Saturday. Josh Johnson is on the 49ers’ practice squad.

Beathard has appeared in four games this season, including the Cowboys game when he replaced Mullens. Against Dallas, he 5 of 7 passes for 100 yards and a touchdown, including a 49-yard Hail Mary TD pass to Kendrick Bourne on the final play of the game. In four games, Beathard is 28 of 45 for 332 yards with two TDs and no interceptions. Mullens was 211 of 326 for 2,437 yards with 12 TDs and 12 interceptions in 10 games, including eight starts.

Shanahan also said tight end George Kittle could return to play this week. Kittle has been out since Week 8 with a broken foot and returned to practice last week. Also, running back Raheem Mostert will miss the final two games of the season due to an ankle injury.

The 49ers (5-9) were eliminated from playoff contention with the loss to the Cowboys.

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