SANTA CLARA -- Manny Lawson's injury didn't look serious at first.
The San Francisco linebacker simply seemed to step awkwardly during practice Wednesday and even pedaled a stationary bike afterward.
Thursday morning, the 49ers learned Lawson will miss the rest of the season after tearing the anterior cruciate ligament in his left knee on a non-contact play. Just like that, the team's promising 3-4 defensive scheme is forced to move on without one of its most important parts.
"I was really looking forward to doing big things this year for my team," said Lawson, who feared the worst after hearing his knee pop. "The hardest thing is just knowing I won't be able to be out there practicing and playing in games with the guys."
Lawson was second on the team with 14 tackles while the 49ers got off to a 2-0 start.
Veteran Hannibal Navies and second-year pro Parys Haralson are the top candidates to replace Lawson.
Coach Mike Nolan said he probably won't name a replacement until game time Sunday in Pittsburgh. Brandon Moore, the team's leading tackler last season, also might have a chance to claim Lawson's spot.