Smith, who had taken every snap for the 49ers the past two seasons, was driven hard to the ground on a sack by Seattle's Rocky Bernard on San Francisco's first drive.
Smith spent the rest of the half in the locker room, and the 49ers announced he wouldn't return in the third quarter.
Trent Dilfer took over, taking his first snaps in a regular-season game since playing for Cleveland in 2005. Dilfer went 6-of-11 for 76 yards in the first half, but was sacked five times. And he threw an interception on San Francisco's first play of the third quarter.
Copyright 2007 by The Associated Press