Starting cornerback Tramaine Brock suffered a sprained big toe against the Cowboys, and his status for Sunday’s game against the Bears is up in the air, a league source said.

Both Brock and the 49ers’ other starting cornerback, Chris Culliver, left Sunday’s win over Dallas in the first quarter. Culliver suffered a concussion and must pass the league’s post-concussion protocol before being cleared to play.

Their replacements played well in their absence. Perrish Cox had an interception while rookie Dontae Johnson, a fourth-round pick, batted away two passes. The 49ers also have the option of using Chris Cook, who is familiar with the Bears from his four-year stint with the Vikings. Cook was a healthy scratch in Dallas.

Another defensive back, Jimmie Ward, also was suspected to have suffered a concussion. Ward is the team’s first option at nickel cornerback. Cox is the top replacement at that position -- and with LaMichael James gone, at punt returner -- if Ward cannot play.

“Talk about the versatile football player,” Jim Harbaugh said of Cox. “He does that – corner, nickel, punt returner, special teams. That’s very valuable.”

Brock had an xray Sunday at the stadium that showed no fractures. He returned for a few plays at the end of the game when Ward’s injury left the 49ers with too few cornerbacks.