Beathard, who was listed as questionable heading into the game because of a wrist injury, will serve as Mullens' backup.
Mullens was promoted from the practice squad to the active roster after starter Jimmy Garoppolo was lost for the season with a torn ACL. He hasn't seen an in-game snap since taking over as the 49ers' No. 2 signal-caller. Mullens gets the start on the same day San Francisco waived journeyman quarterback Tom Savage.
ESPN first reported the development.