SAN FRANCISCO (AP) -- The switch is on again.

Desperate to shore up a secondary hit for a league-high 20 touchdown passes, the San Francisco 49ers are changing cornerbacks for the fifth time in six games in hopes they can stand up Sunday to the St. Louis Rams' high-flying attack.

Ramos McDonald and Monty Montgomery, released by other clubs and claimed off waivers earlier this season, become the sixth and seventh players to start at cornerback for San Francisco.

They replaced R.W. McQuarters and rookie Pierson Prioleau. Darnell Walker, Craig Newsome, and Mark McMillian also have started games at cornerback for the 49ers (3-6). McMillian was waived three weeks ago by San Francisco, which is trying to halt a five-game losing streak, its longest since 1980.

"The carousel continues," 49ers defensive coordinator Jim Mora said. "Every time you make a change you hope and anticipate that's a change you can stick with. Obviously, that hasn't been the case."

San Francisco's secondary has been vulnerable all season but no team exposed its weakness like the Rams (7-2). Kurt Warner ripped the 49ers for five touchdown passes in a 42-20 victory Oct. 10 that snapped the Rams' 17-game losing streak to San Francisco and signaled a shift in power in the NFC West.

"We wanted to come out and just throw, throw, throw against the 49ers and we did that from start to finish," said Isaac Bruce, whose four touchdown catches in the previous meeting prompted San Francisco's first secondary shakeup.

Bruce sees no reason why the Rams, seeking their first win in San Francisco since 1990 and their first season sweep since 1980, can't pass on the 49ers again on Sunday.

"I expect a lot of great things to happen," he said.

"We'd be happy to throw on them. We're just going to do what they give us," added Warner, whose 26 TD passes and 117.8 passer rating top the NFL.

Mora, whose defense is giving up more yards passing than any other, said opposing offenses are completing throws much too easily against San Francisco defenders.

"What we're trying to do is to get guys in there who can make plays," Mora said. "We haven't been winning at the ball enough. We're trying to get guys who can win at the moment of truth."

McDonald, who had five career starts before being released by Minnesota a month ago, said Bruce, Torry Holt and Az-Zahir Hakim put incredible pressure on a secondary.

"They're causing problems for everybody and they've got a good quarterback throwing the ball to them," he said.

Montgomery, who had eight career starts for Indianapolis before being cut in early October, said he and McDonald can't afford to feel intimidated in any way.

"If you think like that, you're already a step behind," he said. "It's been a revolving door but you just walk through that door and go out there and do the job."

San Francisco's problems go well beyond a porous secondary, however.

The 49ers have just one win in six games since Steve Young went down with a concussion, and in the last three games, they've failed to score an offensive TD. That's the first time that's happened in franchise history. A loss in the final seven games would also mean San Francisco's run of 16 straight 10-win seasons would end.

"When you take the No. 1 guy that was responsible for winning out of there, you're not nearly as good a team," Rams coach Dick Vermeil said. "You just can't take Steve Young out of your lineup. Not everybody is as fortunate as I am. Not everybody has Kurt Warner." Warner, a former Arena League quarterback, stepped in after Trent Green went down with a season-ending knee injury in an exhibition game.

Steve Stenstrom will make his second straight start at quarterback for San Francisco since taking over for Jeff Garcia.

"We've got to get our offensive problems solved this week in terms of getting the ball in the end zone," Stenstrom said. "We've got to put some points on the board. We've got a lot of talent and we are capable of coming out and putting this thing together."

San Francisco has managed just six points in each of its last two games but the Rams, who enter favored by a touchdown, are not about to take anything for granted.

"They can still do damage," Rams defensive end Kevin Carter said. "We're by no means fooled by the absence of Steve Young."

ST. LOUIS Season schedule and results

SAN FRANCISCO Season schedule and results

September 12 Baltimore W 27-10 September 26 Atlanta W 35-7 October 3 @Cincinnati W 38-10 October 10 San Francisco W 42-20 October 17 @Atlanta W 41-13 October 24 Cleveland W 34-3 October 31 @Tennessee L 21-24 November 7 @Detroit L 27-31 November 14 Carolina W 35-10 November 21 @San Francisco 4:15pm ET November 28 New Orleans 1:00pm ET December 5 @Carolina 1:00pm ET December 12 @New Orleans 1:00pm ET December 19 NY Giants 1:00pm ET December 26 Chicago 1:00pm ET January 2 @Philadelphia 1:00pm ET

September 12 @Jacksonville L 3-41 September 19 New Orleans W 28-21 September 27 @Arizona W 24-10 October 3 Tennessee W 24-22 October 10 @St. Louis L 20-42 October 17 Carolina L 29-31 October 24 @Minnesota L 16-40 November 7 Pittsburgh L 6-27 November 14 @New Orleans L 6-24 November 21 St. Louis 4:15pm ET November 29 Green Bay 9:00pm ET December 5 @Cincinnati 1:00pm ET December 12 Atlanta 4:15pm ET December 18 @Carolina 4:15pm ET December 26 Washington 8:20pm ET January 3 @Atlanta 9:00pm ET

Records source: STATS, Inc.

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