San Francisco replaced Lawson on its 53-man roster by signing
fullback Zak Keasey from the practice squad before Sunday's game at Pittsburgh.
Lawson, a second-year pro and a starter at outside linebacker in the 49ers' 3-4 defense, was hurt while changing directions on a noncontact play in Wednesday's practice.
He was second on the team with 14 tackles during San Francisco's
2-0 start. Coach Mike Nolan hasn't announced who will replace Lawson in the 49ers' starting lineup.
Keasey spent parts of the last two seasons on the 49ers' practice squad. The former Princeton linebacker is a favorite of Nolan, and he could get playing time against the Steelers if backup tight end Billy Bajema isn't healthy.
Bajema, a blocking specialist whose ankle injury kept him out of last week's game, practiced at full strength Friday along with receiver Darrell Jackson, who sat out two days of workouts this week with a strained back. Both are probable for Sunday's game, along with center Eric Heitmann (ankle), safety Michael Lewis (ribs) and receiver Arnaz Battle (groin).
Rookie receiver Jason Hill, who hasn't played yet for San Francisco, was added to the injury report as questionable with a hamstring injury. He didn't practice Friday.