SANTA CLARA, Calif. — Shaun Hill was promoted to the San Francisco 49ers' starting quarterback Monday night, replacing turnover-prone J.T. O'Sullivan in the first big personnel move of coach Mike Singletary's tenure.

Hill won the first two starts of his career late last season after spending his first five NFL campaigns as a seldom-used backup or third-stringer. He lost a three-way competition with O'Sullivan and Alex Smith during training camp this summer, but O'Sullivan has struggled with 11 interceptions and 11 fumbles in 7› games.

"We just felt like right now, Shaun gave us the best chance of winning," Singletary said in the parking lot of the 49ers' training complex before leaving for the day. "Some of the things that have happened with the turnovers and all that, J.T. just needed to take a little time and gather himself."

Hill replaced O'Sullivan for the final play in the first half of San Francisco's 34-13 loss to Seattle on Sunday after O'Sullivan fumbled on the 49ers' first two possessions and later threw an interception that was returned 75 yards for a score. Hill played the entire second half, going 15-of-23 for 173 yards and a touchdown pass to Jason Hill in a typically efficient performance.

Singletary said offensive coordinator Mike Martz was fine with the decision. Many observers thought O'Sullivan won the preseason quarterback competition largely because of his familiarity with Martz's schemes after they spent last season together with the Detroit Lions.

"If he had done a good job of managing the game, then we wouldn't be talking about what we're talking about right now," Singletary said when asked earlier Monday about O'Sullivan's job security. "J.T. sometimes, like a lot of quarterbacks, sometimes you get yourself in a situation where you want to win, and sometimes you do too much."

Although Hill doesn't have spectacular arm strength, he excels in the game-management skills prized by Singletary. Hill got a two-year contract from the 49ers after his solid play last December, but was beaten out by O'Sullivan, a journeyman on his ninth stint with eight NFL teams during his seven-season career.

"I look at the quarterback position, (and) it's kind of quirky," Singletary said. "It's kind of crazy how you try and figure it out. You just need a football player. All of the other stuff is just for the scouts. You need a football player who can do whatever you need to do for the team to win."

Smith, the No. 1 overall pick in 2005, went on injured reserve shortly before the regular season began with another shoulder injury. He still attends meetings and games with the 49ers, but is almost certain to be cut in the offseason if he doesn't take a steep pay cut.