HOUSTON (AP) -Alex Smith has replaced Shaun Hill at quarterback for the San Francisco 49ers in the second half of their game at Houston.

Hill completed 6 of 11 passes for 45 yards in the first half. San Francisco mustered only 50 total yards and three first downs in the half and trailed 21-0 at the break Sunday.

Smith, the top overall pick in 2005, hadn't played since Nov. 12, 2007. He injured his shoulder in practice three days before last season's opener and was placed on injured reserve.

This year, Hill beat out Smith for the starting job in training camp.

Smith led San Francisco on a five-play, 63-yard touchdown drive on the 49ers' first possession of the third quarter. He threw a 29-yard touchdown pass to Vernon Davis, his first TD pass since Oct. 28, 2007.