SAN FRANCISCO (AP) — The San Francisco 49ers are down a key member of their talented defense heading into Week 1 after placing outside linebacker Parys Haralson on season-ending injured reserve Monday.

San Francisco also signed linebacker Clark Haggans to a one-year contract to help fill the void as the defending NFC West champions prepare for Sunday's season opener at Green Bay.

The Niners announced the roster moves during an off day Monday without providing details on Haralson's injury, though he is believed to have hurt his left arm during the first half of a preseason win at Denver on Aug. 26. He didn't play in Thursday night's exhibition finale, a 35-3 win over the San Diego Chargers at Candlestick Park.

Afterward, all coach Jim Harbaugh would say when asked about Haralson's status was that "he's working through something" — a common, vague phrase used by the reigning NFL Coach of the Year when it comes to acknowledging injuries.

Haralson had been among the 11 returning starters for 2012 on Vic Fangio's defense that ranked No. 1 at stopping the run during a resurgent 13-3 season. But he was set to begin the season as a backup to Ahmad Brooks and Aldon Smith, who set a rookie franchise record with 14 sacks last year.

The 28-year-old Haralson, San Francisco's fifth-round draft pick in 2006 out of Tennessee, started all but one game over the past three seasons: 16 last year, 15 in 2010 and 16 in '09. He recorded 30 tackles, three forced fumbles and two sacks last year as San Francisco snapped an eight-year playoff drought.

Haggans, a 12-year NFL veteran, started every game in 2011 for the Arizona Cardinals, who released him on Friday's cutdown day. He had 54 tackles, three sacks, three passes defended, one forced fumble and a fumble recovery last season.