Kwon Alexander was a massive boost for the San Francisco 49ers defense this season. The energetic linebacker suffered a torn pectoral during the fourth quarter of Thursday night's win over the Arizona Cardinals.

The 49ers placed Alexander on injured reserve today, officially ending his season — his first with San Francisco.

The team also promoted linebacker Elijah Lee from the practice squad, signed linebacker Christian Sam to the practice squad, and opened the practice window for tight end Garrett Celek, who remains on the Reserve/Physically Unable to Perform list.

Alexander entered free agency in March, recovering from a torn ACL suffered last year with the Tampa Bay Buccaneers. San Francisco felt he was the perfect replacement for troubled linebacker Reuben Foster, who was waived last season after several questionable off-the-field decisions.

Alexander was phenomenal for the 49ers this season and showed no signed of being slowed by the ACL injury. He finished the year with 34 combined tackles, 0.5 sacks, four passes defensed, an interception, and a forced fumble through eight starts.

"It's going to be a tough loss," head coach Kyle Shanahan told reporters on Friday before learning of the severity of the injury. "Hopefully, it's not for the whole season, but if it is, we'll deal with it."

Dre Greenlaw will fill in for Alexander for the remainder of the season, said Shanahan.

Alexander, a Pro Bowl linebacker in 2017, was in his fifth NFL season and spent his first four years with Tampa Bay. He has recorded 414 combined tackles, 7.5 sacks, 26 passes defensed, seven interceptions, a touchdown, and seven forced fumbles through his five NFL seasons.

The 49ers initially signed Lee from the Minnesota Vikings practice squad on September 13, 2017. He was waived during the 53-player roster cutdown on August 31 and then signed to the practice squad the next day, where he has remained all season.

Lee recorded 65 tackles, a sack, two passes defensed, and a forced fumble through 16 games and five starts with San Francisco last season. He played college football at Kansas State and entered the NFL as a seventh-round pick in 2017 by the Vikings.

Celek was originally placed on the Reserve/Physically Unable to Perform list on August 31.

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