SANTA CLARA, Calif. - The San Francisco 49ers have won all eight of the games they've played this season, but they did suffer a significant loss in their latest victory.

Linebacker Kwon Alexander, the team's prized offseason free-agent addition, suffered a torn pectoral in Thursday night's win against the Arizona Cardinals, a source told ESPN's Adam Schefter on Friday afternoon.

Alexander is expected to miss the rest of what is shaping up as a season full of potential for the Niners.

I'm built for this shhh! It's all part of the game! I sacrifice everything when I step in between the white line! Ima be Legendary! Everything happens for a reason! I'll be back ‼️

— Kwon Alexander (@kwon) November 1, 2019

Alexander suffered the injury late in Thursday's win and was declared out for the rest of the game soon after getting to the sideline. After the game, Niners coach Kyle Shanahan said Alexander would have an MRI on Friday morning but suggested there was fear that it was a pectoral injury.

In the locker room, Alexander seemed to be in good spirits, did not have his arm in a sling and was able to offer at least some range of motion when talking with team medical personnel. Despite some initial optimism based on that, Shanahan and the Niners remained concerned until the MRI results came in.

Shanahan had a conference call with Bay Area media Friday morning while Alexander and tight end George Kittle were getting MRIs. Kittle suffered knee and ankle injuries against the Cardinals and played through them for most of the game before sitting out the final few drives.

During the call, Shanahan was optimistic that there'd be good news on Kittle but not so much when it came to Alexander.

"Kwon's been great for us this year on the field and as a leader," Shanahan said Friday morning. "I'm a lot more nervous about the MRI coming back today.

"He's been a great addition for us. It's going to be a tough loss. Hopefully it's not for the whole season, but if it is, we'll deal with it. I'm really hoping it's not."

Some of Alexander's teammates reacted to the news on Twitter.

Heartbroken for my brother @kwon ..... this game put you through it man

— Richard Sherman (@RSherman_25) November 1, 2019

.@kwon We gone hold it down for you bro. Going to take all of us to match the energy you bring us pic.twitter.com/0xQssumock

— Arik Armstead (@arikarmstead) November 1, 2019

With the tests confirming Shanahan's worst fears, the Niners will now be without a player who had quickly become one of their emotional leaders in addition to adding some much-needed speed and athleticism to the linebacker corps.

In most of eight games, Alexander had 34 tackles, an interception, a half of a sack, a forced fumble and four passes defended.

"Every time you have a guy go down, you're very worried for him," DeForest Buckner said. "The best thing that Kwon can do for us and himself right now is to get healthy and figure out what's going on."

The 49ers will turn to rookie Dre Greenlaw to fill the large void left behind. Greenlaw stepped in for Alexander against Arizona after moving to Alexander's "Will" linebacker spot from his usual spot at "Sam" linebacker. Greenlaw finished with three tackles and had his first career sack.

Veteran Mark Nzeocha would likely step in to Greenlaw's spot at "Sam" with Azeez Al-Shaair offering depth. Shanahan said practice squad linebacker Elijah Lee could get promoted to offer additional help.

While the Niners must deal with losing Alexander, they are expecting better news on Kittle.

Early in Thursday's game, Kittle took a helmet to his left knee and briefly departed before returning. After the game, he didn't seem too concerned that he had a serious injury.

"I know once something happens, I try to push through as much as you can," Kittle said. "It comes to the point where if I feel like I'm a liability to the team and I'm not the best man for the job then I definitely have to, you know. I just talked to my coach about it, and we made a decision about it."

Because the 49ers played on Thursday, Kittle will have some additional time to heal. San Francisco doesn't play again until Nov. 11, when it hosts the Seattle Seahawks on Monday Night Football.

Shanahan was uncertain but hopeful that Kittle will be available for that game.

"I know he played through a lot of stuff and it's going to be tough on him this week," Shanahan said. "But I feel pretty good that it's not going to be a long-term thing and hopefully we'll get him back this week."