NBC Sports Bay Area reports that Mullens will likely require surgery, which would end his season.
Moments after the report, head coach Kyle Shanahan confirmed the injury on Tuesday and said that Mullens would not play again this season. Tommy John surgery is an option for the quarterback, who is still being evaluated.
"A pretty serious injury," Shanahan said. "We'll let the swelling go down before we exactly decide what to do but he'll be out [for the rest of the season in addition to RB Raheem Mostert]."
Mullens finished Sunday's game 21-of-36 for 219 yards with two touchdowns. He also tossed two interceptions and lost a fumble, providing the Cowboys with enough points off turnovers to earn the upset victory. Mullens was once again among Pro Football Focus' lowest-graded 49ers players for the week.
Beathard finished the game 5-of-7 for 100 yards and completed a Hail Mary touchdown pass in the game's closing seconds. He will likely start on Saturday against the Arizona Cardinals at State Farm Stadium. The team did open the practice window for quarterback Jimmy Garoppolo on Tuesday. However, Shanahan added that he would be very surprised if Garoppolo played on Saturday.
After this weekend's game, San Francisco will finish out its season by hosting the Seattle Seahawks at State Farm Stadium on January 3.
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