Trent Dilfer will start for the San Francisco 49ers on Sunday in place of quarterback Alex Smith, who finally acknowledged a serious arm injury has been affecting his play. Dilfer, the 14-year veteran who struggled through three straight losses while filling in for Smith, got the assignment Thursday from Coach Mike Nolan. Smith and his coach had a lengthy meeting Wednesday following the quarterback's public disclosure that his recently separated right shoulder led to a forearm injury that prevents him from throwing well. Smith and Nolan both had previously denied Smith's arm injury was causing his poor play. So the 49ers (2-7) will turn to Dilfer for their home game against the St. Louis Rams (1-8).