SANTA CLARA — The San Francisco 49ers will have four new starters in their secondary for Sunday's season opener, and not one of them started a game for the team last year.

Three of them weren't even with the Niners in 2010. But the revamped unit has worked well together this summer and is eager to take on the responsibility that comes with holding up the back end of San Francisco's defense.

"It's on us as a secondary," strong safety Donte Whitner said Friday after the 49ers conducted their final practice before hosting defending NFC West champion Seattle. "We understand that we're going to have to play well to make sure our defense is a really good defense."

San Francisco finished 24th in the NFL last season in passing defense. Whitner is one of two former high first-round draft choices the 49ers brought in as free agents this summer to upgrade the unit, joining left cornerback Carlos Rogers as the new leaders of the secondary.

Another newcomer, veteran Madieu Williams, is likely to start at free safety in place of Dashon Goldson, who hasn't practiced since last week due to a sore knee and has ruled himself out of Sunday's game.

Tarell Brown is slated to start at right cornerback in place of Shawntae Spencer, whose string of 32 consecutive starts in San Francisco's secondary is in jeopardy.

Spencer recently returned to practice after missing most of training camp and all four exhibition games with a hamstring injury. Brown was San Francisco's nickel back last season and has been playing with the first unit since mid-August.

"We need that group to play well together and that will be an important unit in this game," coach Jim Harbaugh said.

"You strive to have them all working in tandem. It's important that they're all on the same page and they're all communicating," he added. "That's so important on the back end."

Whitner, selected by Buffalo as the No. 8 overall pick of the 2006 NFL draft, has taken on the lead role as quarterback of the unit and will call out plays for the secondary.

That's one reason the 49ers signed Whitner to a three-year, $11.75 million deal on Aug. 4. Another reason is his physical style around the line of scrimmage. Whitner led all NFL safeties and ranked fifth in the league last year with 140 tackles.

Rogers, the No. 9 overall pick of the 2005 draft, joined the 49ers on a one-year deal Aug. 2 and has settled in as San Francisco's top cover cornerback.

Williams, Cincinnati's second-round pick in 2004, signed a one-year deal with the 49ers on Aug. 1 and will hold down free safety while veterans Goldson and Reggie Smith continue their recoveries from knee injuries.

Goldson, who re-signed with the 49ers on Aug. 9, has started San Francisco's past 32 games at free safety.

Smith, who started the final seven games last year at strong safety, got off to an impressive start this summer as the starting free safety before being sidelined by ligament damage that required surgery.

"We're a unique group," Whitner said.

"Everybody has different attributes that they bring to the secondary, and we've jelled fairly quickly back there. We're going to be pretty good. I feel like we're ready and we're looking to go out and put on a good show Sunday."