The San Francisco 49ers have a glaring hole in their defense, following the Week 9 season-ending pectoral injury suffered by veteran linebacker Kwon Alexander against the Arizona Cardinals on Thursday night.

According to NBC Sports Matt Maiocco, the Niners already know who'll they'll tab to fill in for Alexander: rookie linebacker Dre Greenlaw.

"Dre Greenlaw is the choice to replace Alexander in the starting lineup at weakside linebacker," Maiocco wrote in his Saturday mailbag. "The 49ers' linebacker positions are more interchangeable than they've been in the past, and Greenlaw was the starter at strongside linebacker in the team's base defense. That position is the first to exit the field when the 49ers go to their nickel defense.

"Greenlaw is more similar to Alexander with his ability to run and hit. Alexander will be difficult to replace due to his play-making ability, knowledge of the game, professionalism and attitude. Greenlaw has the physical skills, so his biggest challenges will be from the shoulders up."

The 49ers used their fifth-round pick in this year's NFL Draft to select Greenlaw out of Arkansas. He rose quickly up the ranks during training camp and into the preseason, allowing the Niners to part ways with the veteran, Malcolm Smith, during roster cuts. Yet there is the obvious inexperience Greenlaw currently possesses, which is something defensive coordinator Robert Saleh will have to work around for the remainder of the season.

If anything, Greenlaw should get some extra reps in a role he's more familiar with than the SAM spot.

Maiocco also reported San Francisco will likely promote 23-year-old linebacker Elijah Lee off the practice squad once Alexander heads to injured reserve. Lee, whom the 49ers signed off the Minnesota Vikings practice squad in 2017, started five games last season and possesses excellent special teams abilities.

Lee, Mark Nzeocha and the rookie Azeez Al-Shaair are candidates to take over Greenlaw's previous spot at SAM for base downs.
  • Peter Panacy
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    Peter Panacy has been writing about the 49ers since 2011 for outlets like Bleacher Report, Niner Noise, 49ers Webzone, and is occasionally heard as a guest on San Francisco's 95.7 FM The Game and the Niners' flagship station, KNBR 680. Feel free to follow him, or direct any inquiries to his Twitter account.