NaVorro Bowman is done for the season because of Sunday's left Achilles injury, the 49ers announced.
Bowman underwent an MRI on Monday while the 49ers began a light workout, a day after the star linebacker got hurt in a 24-17 loss to the Dallas Cowboys.
The 49ers are still awaiting the verdict of an MRI on rookie defensive tackle DeForest Buckner, who sustained a left-foot injury in the final two minutes of Sunday's loss.
Bowman, 28, was breaking toward the line of scrimmage when he fell to the grass and grabbed at his left Achilles. The 49ers led 17-14 at the point in the third quarter, but the Cowboys proceeded to storm to a 24-17 comeback win.
Nick Bellore, known mostly as a special-teams ace in his six-year career, replaced Bowman for Sunday's final 29 defensive snaps. Gerald Hodges, who started next to Bowman the first three games, did not see any action on defense and was relegated to special-teams duty, as was Stanford product Shayne Skov.
Michael Wilhoite started in place of Hodges on Sunday and was one of two defenders who played all 76 snaps, with safety Eric Reid being the other.
Wilhoite, Bellore, Hodges and Skov will key the linebacker corps if no reinforcements are added. Bowman played 251 of 260 snaps this season prior to his injury, with the only nine he missed coming in the previous loss at Seattle.
"If NaVorro is out for this week or an extended period of time it'll be really those four that you will see in there: Bellore, Wilhoite, Hodge, and Skov," Kelly said prior to the MRI results. "Those are the four we've got."
Regarding Skov, Kelly said the Stanford product can play both inside linebacker roles, as can Wilhoite: "Whenever you're the backup you have to be able to play multiple positions just because we just can't say 'Hey, two guys are down, Shayne can't play because he doesn't know how to,' real smart kid, Stanford kid. He can play both positions."
When Bowman missed the 2014 season recovering from knee reconstruction, Wilhoite started all 16 games as a replacement next to Patrick Willis, who succumbed to a career-ending toe injury after six games and was replaced by Chris Borland.
