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SANTA CLARA -- Cornerback Tramaine Brock's status for the 49ers' game Sunday against the Chicago Bears is in serious question after an MRI revealed he has a sprained big toe, a source said.
Brock started the season opener against the Dallas Cowboys but was forced out of the game early in the first quarter after he hyperextended his toe -- a common NFL injury known as "turf toe."
He returned late in the game after nickelback Jimmie Ward was sidelined with a concussion. Brock played a total of 10 snaps in the game.
Cornerback Chris Culliver appeared to be doing fine on Monday after being diagnosed with a concussion. Culliver and Ward must pass the NFL's protocol on concussions before they can get cleared to play.
Perrish Cox is in line to start against the Bears if Brock is unable play.
Dontae Johnson and veteran Chris Cook would serve as the backups. Chicago, which opened the season with a 23-20 loss to the Buffalo Bills in overtime, features a group of wide receivers that includes Brandon Marshall, Alshon Jeffrey and Santonio Holmes.