Beathard, who was listed as questionable this week because of the injury, has made enough progress since Sunday to be able to serve as the backup while Mullens makes his NFL debut.
The 49ers released third-string quarterback Tom Savage on Thursday afternoon in order to add more depth at other positions with injury issues. The Niners promoted cornerback Emmanuel Moseley from the practice squad and linebacker Dekoda Watson and strong safety Marcell Harris were activated from injured reserve.In what figures to be the final "Battle of the Bay" between the teams before the Raiders move to Las Vegas, Mullens will make his first regular-season appearance in prime time.
The Niners originally signed Mullens out of Southern Miss as an undrafted free agent in 2017. He spent that year on the practice squad and started this season on the practice squad as well. After starting quarterback Jimmy Garoppolo suffered a season-ending ACL injury in Week 3, the 49ers promoted Mullens to the active roster to back up Beathard.
To this point, Mullens' NFL playing experience has been limited to the preseason. He was 31-of-43 for 396 yards with a touchdown and three interceptions and lost two fumbles in the 2018 preseason.
"I thought he played well in the preseason," coach Kyle Shanahan said this week. "He did good. Nick comes in there, moves the chains, competes hard, made some plays with his legs and his arm. Guys believe in him and he's as competitive and confident as a guy I've been around."
Mullens, Southern Miss' career leader in passing yards (11,994) and touchdowns (87), played in a spread offense in college, but Shanahan said Mullens went to great lengths to adjust to his pro-style offense. Those efforts included going to YouTube for tips on how to take snaps from under center as he went through the pre-draft process.
Shanahan indicated he wouldn't have to pare down his offense for Mullens, though Mullens hasn't taken many live practice reps with the first team because of the short week.
"That's the challenge of being a backup," Mullens said. "You don't get a lot of reps, but you have to keep yourself mentally sharp throughout the week. So, really just playing the game in my head, that's kind of what I do. I go home, see what plays we have in for the week and just play the game in your head before it actually plays out."
Beathard, meanwhile, did all he could to play on a short turnaround after banging his right hand on a defender's helmet in the second quarter of Sunday's loss to the Arizona Cardinals. As recently as Tuesday, he was unable to grip a football, but that improved enough on Wednesday and Thursday for him to at least be available to play against the Raiders.
Had this week's game been played Sunday, Beathard likely would have been ready to start, according to Shanahan.