Cornerback mystery: Who plays vs. Steelers?

Following CB Cary Williams’ fourth-quarter removal in favor of DeShawn Shead Sunday, a position battle has bubbled up in the Seattle Seahawks’ secondary. It may be devoid of animosity, a quiet competition among determined professionals, but the result will be crucial to Seattle’s chances against the Pittsburgh Steelers, who have the league’s most dynamic passing duo.

Coach Pete Carroll said that the competition for the cornerback position opposite Richard Sherman would bring out the best of what the secondary has to offer.

Williams, 30, was benched after the 49ers completed a 36-yard pass to TE Lance McDonald, and he did not return. After the game, Williams admitted it hadn’t been his best day.

“It was just more of a mental thing than anything,” Williams told reporters. “Maybe I was thinking about things too hard, or maybe I was looking into it a little bit different, or maybe my eyes weren’t necessarily in the right place. I don’t know why.

“I felt like I prepared pretty well for it, but for some reason I didn’t go out there and play my best ball today.”

Now, before going against QB Ben Roethlisberger and principal WR targets  Antonio Brown and Martavius Bryant, Shead and Williams have to win the job this week in practice.

Williams said he was trying not to think about it as a competition.

“I think of it as more of an opportunity for me,” he said. “I don’t really think of it as a competition between me and anybody else, I just think it’s an opportunity for me to get better and for me to focus on myself and get back to where I need to be.

“(I’ve got to) compete during the week, compete on Sunday, and just play with a fire and a desire out there that I’ve done in the past.”

At the moment, it seems that Williams, a nine-year vet with his fourth NFL team, is on the outside looking in at the starting job.

Shead, a four-year vet, all with Seattle, said he was just trying to focus on his game.

“I’ve got to go out there and do my best and do what I can do to stay on top and make the most of my opportunity,” said Shead, who graduated from Portland State.

That’s a standard response for a former practice-squad player trying to snag a starting spot, especially in the Seahawks’ large inventory of overlooked talent.

Whoever turns in the better performance Sunday will earn a substantial amount of credibility for the remaining the five weeks of the regular season.

Whoever starts will have hands full against the Steelers. Brown has a league-best six games with 100 yards or more. He is second overall in yards — 1,141 on 79 receptions. Bryant, meanwhile, is averaging 20 yards per catch.

The Steelers are a constant threat to go deep, similar to the Arizona Cardinals, who beat the Seahawks at the Clink, 39-32.

“Everybody can catch, everybody can run, everybody’s good, and Ben’s got all those plays in him,” said Carroll. “He’s a great scrambler inside the hash marks . . . he gives himself a second or third shot at plays, that’s very difficult (to stop).”

If Williams or Shead can limit damage on the side of the field opposite Sherman, who will likely be assigned to hound Brown, the Seahawks improve their shot at beating a team with a winning record — a seasonal first.