San Francisco 49ers linebacker Kwon Alexander will miss the rest of the season after tearing his pectoral muscle.
The 49ers announced the diagnosis Friday, a day after Alexander got hurt in a victory at Arizona that improved San Francisco's record to 8-0.
Alexander has been a key part of one of the NFL's stingiest defenses in the first half of the season with his speed allowing him to cover tight ends and running backs when necessary. He has also taken up a leadership role on the unit that will be hard to fill.
This is the second straight year Alexander went down with a significant injury. He tore his ACL last year in Tampa Bay before signing with the Niners as a free agent in March.
Rookie Dre Geenlaw is expected to take Alexander's spot on the nickel defense with rookie Azeez Al-Shaair and practice squader Elijah Lee in contention to take Greenlaw's spot in base packages.