SANTA CLARA -- If Adam Snyder replaces Mike Iupati because of a sprained knee for the coming weeks, the 49ers better hope pass-protection issues don't flare up like they did to start their final drive Sunday.

Snyder got beat on a twist by Saints defensive end Junior Galette en route to a 9-yard, first-down sack of Colin Kaepernick at the 49ers' 11-yard line.

Iupati left in the third quarter with a sprained medial collateral ligament to his left knee, and there's no known timetable for his possible return, a source confirmed Wednesday.

The 49ers' other top injury concerns are those with cornerback Tarell Brown (ribs) and defensive tackle Ray McDonald (ankle). Brown got hurt Sunday while McDonald missed his first game of the season, and their prognosis is unknown for Sunday's game at Washington.

Because Snyder is their most experienced and versatile backup lineman, he likely will get the call to replace Iupati at left guard instead of Daniel Kilgore, who's developed well but has never started a game in his three seasons.

Snyder took over in the third quarter Sunday when Iupati's troublesome knee got hurt on a Frank Gore run for no gain.

The 49ers have started the same five offensive linemen since the start of last season, a 29-game stretch that includes playoffs. Snyder has started 83 career games, including 13 games at right guard in 2011 for the 49ers and 14 last season with the Cardinals (11 at right guard, three at center).

Joe Looney, a 2012 fourth-round draft pick, still hasn't made his 49ers debut and has been inactive every game this season.


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For more on the 49ers, see Cam Inman's Hot Read blog at blogs.mercurynews.com/49ers. Follow him on Twitter at twitter.com/CamInman.