SANTA CLARA — QB1 Jimmy Garoppolo tore up his knee three games ago. QB2 C.J. Beathard got ransacked Sunday by the Arizona Cardinals. As for QB3, that role gained clarity Monday.

Coach Kyle Shanahan left no doubt that Nick Mullens is the one on-call if Beathard gets hurt for the 49ers (1-4). A QB4 could be coming, but that is only to back up Mullens and ease the workload at practice.

“We know Nick Mullens is our backup to Beathard,” Shanahan said. “So, we’re looking for a guy who would have the chance to come in and play if C.J. went down and Nick was going, so we could have an immediate backup right away.

“You’d love to ideally find that as a practice-squad guy, but we haven’t been able to do that yet.”

They haven’t because this injury-laden team has needed reinforcements at other positions, and this week’s bigger priority is at running back. Auditions were held Monday for free agents, and former Cleveland Browns backup Matthew Dayes was added Tuesday to the practice squad.

Matt Breida will “probably end up being doubtful” for Monday night’s visit to Green Bay, Shanahan said. Breida became the RB1 equivalent upon replacing the injured Jerick McKinnon, Garoppolo’s upcoming partner in knee rehabilitation the next nine months.

Breida has played through injuries all season, but if he can’t go this time, Alfred Morris would be the workhorse at Lambeau Field. Raheem Mostert, whose specialty is special teams, fumbled on his first carry in place of Breida in Sunday’s 28-18 loss to Arizona.

“Raheem is one of the best special teams players in the league and that’s why he’s going to always be up on game day,” Shanahan said. “But he is also our third running back and your third running back can be a very important factor in the game at any moment, the way our backs have struggled to stay healthy through these games.

“Raheem is someone who does have to step it up for us when people go down.”

However the running back hunt progresses, Shanahan said the 49ers would circle back to the quarterback depth chart this week, noting: “We haven’t come to a conclusion yet on the quarterback. We’ll look more into that (Tuesday).” Until Garoppolo’s injury, Mullens toiled on the practice squad since arriving undrafted out of Southern Miss last year.

Speaking of offensive backups, wide receiver Dante Pettis (knee) won’t return to face the Packers, and Marquise Goodwin (thigh, hamstring) might miss his second straight game, too.

In their place, Victor Bolden and Richie James filled in at split end with mixed results, and the same goes for de facto WR1 Pierre Garçon, WR2 Trent Taylor and WR3 Kendrick Bourne.

So, to recap, the 49ers are on the prowl for a few good men, be it a QB4, RB4, RB5, WR4, WR5, WR6. That’s enough to keep fans’ attention, or at least fantasy football owners’ interest.