Seahawks quarterback Matt Hasselbeck, plagued in 2008 by chronic back problems, took a hard shot to the spine from 49ers linebacker Patrick Willis at the goal line.
Hasselbeck seemed to be fine after absorbing the blow, but then he collapsed while walking toward the sidelined, and he left the game.
Seneca Wallace replaced Hasselbeck, and tossed a short touchdown pass to tailback Julius Jones.
The 49ers lead at the half, 13-10. Frank Gore has 126 yards rushing and a touchdown for San Fran.