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49ers lose heart and soul of defense in Alexander
49ers linebacker Kwon Alexander
San Francisco 49ers linebacker Kwon Alexander (56) launches himself toward Pittsburgh Steelers running back James Conner. - photo by Photo by Stu Jossey

SANTA CLARA (AP) — Kwon Alexander wasn’t just the 49ers’ best linebacker. He was the heart and soul of their defense. And now they must replace him with a rookie fifth-round draft pick, Dre Greenlaw.

The 49ers placed the 25-year-old Alexander on injured reserve Tuesday with a torn left pectoral muscle. He suffered the injury last Thursday against the Arizona Cardinals when he hit running back Kenyan Drake violently with his left shoulder, left the game and never returned. Alexander will miss the rest of the season.

“Since Day 1, the defense has been what it is because of him,” starting middle linebacker Fred Warner said Tuesday. “He was playing at a Pro Bowl level, but the things people didn’t notice is what really made him stand apart.”

His statistics tell only part of the story. Alexander had 34 tackles, one forced fumble, one interception and four pass breakups. He also allowed just five catches on 12 targets.

“He brought communication, for one,” Dee Ford said. “He kept us all on one page. He brought energy. He made big plays. He definitely is a pivotal part of our defense.”

Big shoes to fill.

“It’s not my job to fill his shoes,” Greenlaw said Thursday. “It’s my job to do my job. That’s how I look at it. I like to have fun and bring energy. At the same time, I know I can’t let the game get to my head. I can’t go out there and get excited about one play when I still have 20 or 30 other plays. It’s about bringing the energy I’m supposed to bring.”

Greenlaw isn’t totally green, even though he’s a rookie. He has started three games at strong-side linebacker and has recorded 14 tackles and one sack. After Alexander tore his pec against the Cardinals, Greenlaw moved to Alexander’s position, weakside linebacker.

“I’ve had a couple position changes,” Greenlaw explained. “Last game, I had to process things a little differently. I knew what they were doing, the plays they were running, but I was slightly hesitant, more than I usually am, because the last few weeks I’ve been focusing so much at (strong-side linebacker).”

The 49ers need Greenlaw to make a smooth transition, because their run defense needs help. The past four games, the 49ers have allowed 5.5 yards per carry.

“It’s little things,” Warner said, “little assignment miscues, guys not wrapping up here and there. Things we can fix.”

Rookie Nick Bosa believes in Greenlaw. “He’s an unbelievable player,” Bosa said. “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 already has been starting in our base package, so it’s just a little more on his back this week and the weeks to come. I think he’ll be ready for it.”

This week, Greenlaw must prepare to face a Seattle Seahawks offense that ranks eighth in passing yards and ninth in rushing yards. The Seahawks, a run-first team, will test Greenlaw.

“I’m expecting them to come at me, of course,” Greenlaw said, “because I’m a rookie playing a new position.”