The San Francisco 49ers lost a key component of the defense last week after linebacker Kwon Alexander left the Halloween matchup against the Arizona Cardinals with a torn pectoral muscle. The team officially placed Alexander on injured reserve on Tuesday, ending his first season with San Francisco.

"It's definitely a huge, huge loss," rookie defensive end Nick 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 Greenlaw] 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."

Alexander finished the year with 34 combined tackles, 0.5 sacks, four passes defensed, an interception, and a forced fumble through eight starts. More important, he emerged as a vocal leader on defense.

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The 49ers have a next-man-up mentality. The team has lost key starters throughout the season and hasn't seemed to miss a beat. San Francisco is 8-0 after losing both of its starting tackles, fullback, and cornerback. Justin Skule, Daniel Brunskill, and Emmanuel Moseley are names many may not have even known before injuries thrust them into the starting lineup. Each has performed admirably while filling in.

"We have a lot of players who are ready to play, like you saw," Bosa continued. "Skule, Dan, just really didn't miss a beat (while) playing some really big games for us. I mean, the coaching goes along with it too, just how Kyle (Shanahan) calls plays and puts players in the right positions."

Greenlaw, a rookie who impressed during the offseason, has 19 tackles and a sack this season and will see his workload increase moving forward.

"He's an unbelievable player," Bosa said of his fellow first-year defender. "He wouldn't be here and he wouldn't have worked his way up to this position if he wasn't a really good player. He's good under pressure. He's already been starting in our base package so it's just a little more on his back this week, and for the weeks to come, and I think he'll be ready for it."