San Francisco cornerback Perrish Cox’s name didn’t appear at the top of the Saints’ concerns back in April, when New Orleans first learned it would host the 49ers on Sunday.

But more than a half year later, Cox somehow stands among the chief threats the 49ers (4-4) pose to the 12-0 home record the Saints have built up since 2013.

Cox is first on the Niners and tied for second in the NFL with four interceptions. Two of his picks have been late in games and sealed a pair of victories for San Francisco, while another set up a touchdown for his team.

He also has a pair of fumble recoveries (one of which led to another Niners touchdown), meaning Cox is single-handedly responsible for almost half of the 13 takeaways San Francisco’s second-ranked defense had created by the time it set its sights on the Saints.

“At the end of the day, you look at production, ... (and) he’s a guy who has a nose for the football right now,” said Saints quarterback Drew Brees, who’s thrown for 2,524 yards, 15 touchdowns and eight interceptions midway through New Orleans’ season. “Creating turnovers ... (has) always been a big factor in the games that they play, ... but Perrish Cox is having a great year in regards to that.”

Yet none of that is more remarkable than the fact that it was never the plan for Cox to be on the 49ers’ first string. After all, he entered the 2014 campaign with a lone takeaway to his credit.

A 2010 fifth-round draft choice out of Oklahoma State for Denver, Cox played one season with the Broncos. But Denver waived Cox as he awaited trial on a sexual assault charge, for which a jury acquitted him in 2012.

Cox signed with the 49ers days after jurors declared him not guilty. He was a backup on 49ers teams that made a Super Bowl and two NFC championships over the next couple of seasons, and he was projected to again be a reserve this year — until starting outside cornerback Tramaine Brock sustained a turf toe injury in Week 1, and Cox stepped into his spot.

Over the following weeks, Cox made four different quarterbacks very sorry that they tested him with either forced or off-target throws aimed mostly at their top receivers.

“He’s done a fantastic job — he’s been a shining star for our ballclub,” 49ers coach Jim Harbaugh said about Cox during a conference call with New Orleans media Wednesday. “He’s another guy with his effort, his execution and his talent (who is) making the most of his opportunities. I’m very pleased.”

Cox picked off a deep throw from Cowboys quarterback Tony Romo to star receiver Dez Bryant, setting up one of the touchdowns San Francisco scored in a 28-17 win at Dallas in both teams’ season opener.

Cox’s next two victims — the Eagles’ Nick Foles and the Chiefs’ Alex Smith — were attempting to lead game-winning drives late in the fourth quarter. However, Foles tried to squeeze a pass to Jeremy Maclin while the receiver stood at the center of a triangle created by Cox and two other 49ers defenders. Cox intercepted Foles’ throw along one of the sidelines to secure a 26-21 victory for the 49ers at home.

Meanwhile, Smith overthrew Anthony Fasano and inadvertently connected with Cox, whose pick let the 49ers kneel out a 22-17 win in San Francisco against the Chiefs.

Two of Cox’s three remaining takeaways led to 49ers punts in defeats: an interception of Rams quarterback Austin Davis, who underthrew receiver Kenny Britt while rolling out to the right and away from pressure in San Francisco’s 13-10 setback at home last Sunday; and the recovery of a Larry Fitzgerald fumble that stopped the Cardinals five yards shy of the goal-line in a 23-14 loss at Arizona.

But Cox’s second fumble recovery illustrates the kind of uncannily opportunistic year he’s having. On that one, from his own 20, Foles threw a short pass to Eagles tight end Zach Ertz, who was immediately throttled by 49ers safety Antoine Bethea. Ertz fumbled the ball forward, and it fell directly into the grasp of Cox, who had the presence of mind to touch the field with the tips of both of his toes before tumbling out of bounds. The 49ers scored on the ensuing possession.

It might be tempting to write off Cox’s performance this year as a series of fortunate events while pointing to Ertz’s miscue as evidence. That’d be a mistake, though.

Cox is affording himself his luck with excellent coverage. Quarterbacks have registered only 22 completions, 314 yards, a touchdown and a dismal passer rating of 42.3 the 45 times they’ve challenged Cox.

That passer rating is the second-lowest among the 46 NFL cornerbacks who have played at least 75 percent of their team’s defensive snaps this season, according to the website Pro Football Focus.

“The first things we’re always real keen on are ... the level of ball skills players have,” Saints coach Sean Payton said Wednesday. “(With Cox), you have to be precise with your (ball) location (throwing against him).”