Lawson, a starting outside linebacker in the 49ers' 3-4 defensive scheme, went down with the injury during yesterday's practice. The injury didn't initially appear to be serious, and Lawson rode on a stationary bike for the rest of the team's workout.

But tests determined today that Lawson tore his anterior cruciate ligament.

Lawson is second on the team with 14 tackles, showing impressive improvement from his rookie year. He started 11 games last season after San Francisco chose him with the 22nd overall selection, making 65 tackles and 2 1/2 sacks.