Linebacker Kwon Alexander has been having a phenomenal first season with the San Francisco 49ers. That's why when he left with an injury late in the third quarter during the 28-25 win over the Arizona Cardinals on Thursday night, it was so disheartening.

Alexander has quickly become a vocal leader on defense and a significant reason for the unit's turnaround from a lackluster campaign last year.

Alexander underwent an MRI on Friday morning, and Adam Schefter of ESPN reports that the star linebacker and big offseason addition suffered a torn pectoral muscle, ending his season.

Update: The 49ers confirmed the injury on Friday and announced that Alexander will likely undergo season-ending surgery next week.

Head coach Kyle Shanahan admitted to being nervous about the MRI results while speaking with reporters via a conference call on Friday morning.

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"Kwon has been great for us this year on the field and as a leader," Shanahan said. "I'm a lot more nervous about that MRI coming back today. I'm hoping for the best, but I'm a lot more nervous about that one."

The 49ers signed Alexander in March. He has registered 34 combined tackles, 0.5 sacks, four passes defensed, an interception, and a forced fumble through eight starts this season.

"It's going to be a tough loss," Shanahan added. "Hopefully, it's not for the whole season, but if it is, we'll deal with it. But I'm really hoping it's not because he's been a hell of an addition for us."

Shanahan told reporters that rookie Dre Greenlaw will fill in for Alexander, should the veteran linebacker be forced to miss time.

Alexander returned this year and was having a phenomenal season after his 2018 campaign with the Tampa Bay Buccaneers ended early due to a torn ACL.

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