Fill-in 49ers defensive back Cox has become too useful to bench

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The brilliant performance of 49ers cornerback Perrish Cox in a fill-in role this season has inspired a question: What happens to the second-stringer when starter Tramaine Brock returns from a toe injury?

“We’ll figure that out then,” defensive coordinator Vic Fangio said.

Said Cox: “I have no clue at all.”

Posed the same question, however, former 49ers cornerback Eric Davis didn’t hesitate to provide a definitive answer: Cox isn’t going back to the bench.

“He’s making too many plays,” said Davis, 46, an NFL Network analyst who was a two-time Pro Bowler in his 13-year NFL career. “He’s not going to come off the field. The only question is, how are they going to work him in? It’s really that simple. You don’t take production off the field, ever.”

Indeed, Cox, 27, who hadn’t started a game since his rookie season in 2010, has looked more like an All-Pro than an erstwhile backup. He replaced Brock in the first quarter of the season opener and has since collected team-highs in interceptions (two), passes defended (eight) and fumble recoveries (two). Quarterbacks have a 28.6 passer rating when throwing in his direction, the second-lowest figure in the NFL, according to Pro Football Focus.

Davis was the 49ers’ radio analyst from 2011-13, but he didn’t see this coming. Mostly because he didn’t see much of Cox, who signed with the 49ers as a free agent in 2012 and was released three times, twice by the Seahawks, last year. Cox has played 253 snaps in four games this season, four more than he played the previous two years.

“He is playing as sound at the position as anybody in the league,” Davis said. “It’s ridiculous the way he’s playing. He’s taking full advantage. Like a veteran should do and understands how to do, he’s taking full advantage of the opportunity he’s been given.

“Jobs are not won or lost in the NFL. Opportunities are given and taken. And he’s taking advantage of his opportunity to be on the field.”

So who leaves the field if Cox remains? Brock and Chris Culliver aren’t likely to cede their starting roles. Brock signed a four-year, $14 million extension last year and was impressive in his seven starts in 2013. Culliver, a 2011 third-round pick, has been a key part of a defense that’s allowed the fourth-lowest completion percentage (58.1) and fifth-lowest passer rating (77.7) to quarterbacks.

That leaves rookie Jimmie Ward, a first-round pick who has played 57 percent (150 of 263) of the defensive snaps in his role as nickel cornerback. Ward has endured typical welcome-to-the-NFL growing pains: Most notably, he surrendered three touchdowns to wideout Brandon Marshall in a Week 2 loss to the Bears.

Cox has experience in Ward’s spot. Last season, he capably subbed for nickel corner Carlos Rogers in road playoff wins over Green Bay and Carolina.

“With a young player like Ward, that’s probably the first one you’re going to look at,” Davis said. “If you don’t have to immediately put him out there, it’s a luxury. You give him a chance to grow.”

Davis is particularly bullish on Cox because he knows from experience what’s required of cornerbacks in Fangio’s defense. Davis, who played three seasons (1996-98) with Fangio at Carolina, said his former coach seeks versatility and toughness at the position. The 49ers play a mix of man-to-man and zone coverage, and cornerbacks consistently play a role in run support.

Cox, 6-foot and 190 pounds, has shown a willingness to do the dirty work while collecting 10 tackles this season.

“There are times when you have outside contain on a sweep or dive, when a guard or tackle pulls and you’re going to have to take that hit and end up like a bug on a windshield,” Davis said. “But then (linebacker) Patrick Willis can scrape around and make that tackle. It will never show up in the stat book, but you are asked and required to do those things within that defense.”

For his part, Fangio acknowledged Cox has fulfilled his various duties with distinction since he was handed his opportunity. But he didn’t hint at how the 49ers might solve the conundrum when Brock, who hasn’t practiced this week, is fully recovered.

Cox “definitely has played well,” Fangio said. “And his play has merited playing. That’ll be a good problem to have and we’ll solve it.”

Eric Branch is a San Francisco Chronicle staff writer. E-mail: ebranch@sfchronicle.com

Coverage kings

Quarterbacks have a 28.6 passer rating when throwing in the direction of 49ers cornerback Perrish Cox, the second-lowest figure in the NFL, according to Pro Football Focus.

1. Vontae Davis, Colts: 22.6 rating

2. Perrish Cox, 49ers: 28.6

3. Prince Amukamara, Giants: 36.0

4. Chris Harris, Broncos: 41.3

5. Greg Toler, Colts: 47.9

(Minimum 100 snaps)