When the San Francisco 49ers tell you they have faith in someone to replace an injured player, you better believe them. The narrative has come up multiple times this season.

There were doubts when starting left tackle Joe Staley went down, and rookie Justin Skule had to fill in. San Francisco remained undefeated.

There were doubts when starting right tackle Mike McGlinchey went down, and the team turned to relatively unknown backup Daniel Brunskill. San Francisco remained undefeated.

There were doubts when starting cornerback Ahkello Witherspoon was injured, and Richard Sherman tried to convince everyone not to overlook his replacement, Emmanuel Moseley. San Francisco remained undefeated.

Then there was the injury to the team's only fullback, Kyle Juszczyk. This time, "next man up" meant players who typically play other positions filling that hole.

You guessed it; San Francisco remained undefeated.

So when the 49ers say rookie linebacker Dre Greenlaw is capable of filling in for Kwon Alexander, who suffered a season-ending pectoral tear during Week 9, who are we to say otherwise? San Francisco could very well keep rolling.

That's not to say that Alexander won't be missed. It just means that replacing him will be a group effort, as standout rookie defensive end Nick Bosa said this past week. Greenlaw will be a significant part of that effort.

"It's definitely a huge, huge loss," Bosa said on Tuesday. "As a defense, we're just going to have to pick it up. Every single player is going to have to play a little better. Obviously, Dre is going to have to fill in, but it's going to be a group effort to bring that energy because he really does bring the energy for us."

It's not like Greenlaw will be out of his element. He has taken nearly half the snaps compared to Alexander's (173 versus 357) and has been in during base downs. Greenlaw's role will simply be expanded. The toughest aspect of Alexander's game to replace will be his play in coverage.

Defensive coordinator Robert Saleh was asked this week what the 49ers defense will miss the most during Alexander's absence.

"His energy, his juice, obviously his playmaking ability," Saleh responded. "There's going to be a lot to miss, but we've got tremendous faith in Dre Greenlaw and the guys that are going to step up in his place.

"Obviously, it's a loss. But at the same time, this is like I've always said before — it's the NFL. They're not going to wait. No one's going to feel sorry for you. They never ask how; they ask how many, and Dre's got to step in and make sure that there's no misstep once he steps in there."

Greenlaw has 19 tackles and a sack through eight games this season. Head coach Kyle Shanahan shared his thoughts on the rookie's practices this week as he prepared for his expanded role.

"He looked good," Shanahan told reporters on Saturday. "He's played a lot in base this year, so he's been out there. The game never seems too big for him. He's used to being out there. He's just going to be out there in all situations now.

"He had a good week of practice. It was nice to get a bunch more reps for him, especially in the nickel situations. I'm not worried about Greenlaw, excited to see him play."

The 49ers and Seattle Seahawks will meet on Monday night at Levi's Stadium for an NFC West showdown. Kickoff is scheduled for 5:15 p.m. PT.