SANTA CLARA, Calif. (AP) -- The San Francisco 49ers have been losing key players in their secondary all year yet their defense hasn't skipped a beat.
That continuity gets another tough test this week without starting right cornerback Tarell Brown.
Brown left last week's game at New Orleans with a rib injury and hasn't practiced this week. He's unlikely to play Monday night when the 49ers (6-4) look to end a two-game losing streak against the Washington Redskins, the first game missed by a regular starter in San Francisco's secondary since the 2011 season.
But the 49ers are confident nickel back Tramaine Brock will step up in Brown's place, just like Brock stepped in for injured nickel back Nnamdi Asomugha earlier this season and became an impact player on the NFL's seventh-ranked defense who leads San Francisco with four interceptions.
Brock received a four-year contract extension Friday night through the 2017 season.
Now, Brock is in line to make just his second NFL start. His first came in October when the 49ers opened the game with five defensive backs against the Houston Texans. Brock responded with two interceptions in that game, returning the first 18 yards for a touchdown on the game's third play to spark San Francisco toward a 34-3 victory.
Brock played sparingly in San Francisco's first three games, but he has seen regular action and become a playmaker since then. Brock ranks second in the NFL in interceptions and his nine pass breakups are tied for the team lead with Brown.
''He's gone from not playing, to being in the nickel, which is a part-time job, to now it's amped up another level that he's in there every play,'' defensive coordinator Vic Fangio said Friday.
The 49ers have adjusted well to change in their secondary since losing free safety Dashon Goldson to free agency in March. Goldson was a first-team All-Pro last season who led the 49ers in interceptions and was fifth on the team in tackles.
San Francisco drafted Eric Reid in the first round to replace Goldson, and Reid has quickly developed into one of the NFL's top rookie defenders this season, recording three interceptions and two fumble recoveries. He also hasn't missed a start despite leaving two of San Francisco's first nine games with concussions.
The 49ers lost cornerback Chris Culliver, San Francisco's regular nickel back in 2011-12, to a season-ending knee injury during a training camp practice in August. Asomugha took Culliver's place when the season began and was off to a good start in the role before he injured a knee in Week 3. Brock took Asomugha's place in the nickel and played so well that the 49ers released the veteran earlier this month.
Brock replaced Brown when the latter was injured during the second quarter of San Francisco's 23-20 loss to the Saints. Recently activated cornerback Eric Wright took Brock's place in the nickel and ''did well considering the circumstances,'' Fangio said, in the veteran's first action with the 49ers.
Wright is slated for nickel back duty against the Redskins. Starting left corner Carlos Rogers and undrafted rookie Darryl Morris are the other healthy cornerbacks on the San Francisco roster.
''Now somebody has to step up again,'' Reid said. ''We have a couple of guys in Eric Wright and T-Brock, so we just have to fill the gap and keep moving forward and trusting these guys that are going to step up and play.''
Brown had started 47 consecutive games for the 49ers, who have become very efficient this season at replacing key players on defense with reserves who perform effectively.
That has been particularly true in the secondary, where Brown, Rogers, Reid and strong safety Donte Whitner have given San Francisco one of the NFL's most cohesive starting units.