SANTA CLARA -- The 49ers are placing linebacker Ray-Ray Armstrong on injured reserve, the 49ers announced on Tuesday morning.
He sustained a pectoral injury on Sunday in Carolina. He underwent an MRI examination on Monday that revealed the likely season-ending damage. Armstrong is expected to undergo surgery.
"It sucks," 49ers defensive coordinator Jim O'Neil said. "But I've said all along we're going to need (Michael) Wilhoite, we're going to need Gerald (Hodges) to help us win football games. And those guys will step up and do a great job in that role."
Hodges started the first two games alongside Navarro Bowman at inside linebacker. Armstrong saw considerable action -- 27 defensive snaps -- before sustaining his injury in the third quarter. Armstrong saw more playing time than Hodges in Week 1.
He recorded his first career interception in the 49ers' Monday night opener against the Los Angeles Rams. Armstrong also had a fumble recovery in the 49ers' loss Sunday to the Carolina Panthers.
"I thought he was playing his butt off," O'Neil said. "He was playing a really good game for us. He made some splash plays in that Carolina game. It's one of those unfortunate deals that happen in this league. You can't worry about it. You have to move on."
The 49ers are expected to announce a corresponding roster move later in the week.