Captain Munnerlyn was entering his third NFL season when new Carolina Panthers coach Ron Rivera approached him with a challenge.

Munnerlyn had lined up almost exclusively as an outside cornerback throughout high school, a standout collegiate career at South Carolina and his first two years in the league. But after Richard Marshall departed in free agency, the Panthers needed someone to man the inside in the nickel.

"I'm sitting there like, 'Maaaaaan!' " Munnerlyn recalled, rolling his eyes demonstratively.

Munnerlyn had heard from veteran defensive backs about how difficult the nickel job was. But when Rivera gave him a hard sell with talk about Munnerlyn having the potential to be one of the league's best at sticking to shifty slot receivers, Munnerlyn reluctantly agreed.

Over the three seasons that followed, Munnerlyn grew into the role and in time became one of the league's better slot defenders, though even today he readily admits the role still isn't an easy one.

"Man, it's tough,'' Munnerlyn said. "I used to joke with my teammates that you should get paid a whole heck of a lot of money to play nickelback."

It might not have been a whole heck of a lot of money by current NFL standards, but the Vikings this offseason signed Munnerlyn to a significant sum — a three-year deal with $7 million guaranteed that can max out at $15 million — to fill the void left when Antoine Winfield, an exceptional cover man in the slot, reached the twilight of his career and was not brought back in 2013 by General Manager Rick Spielman.

Leaguewide, the use of three-receiver sets continues to grow year after year. Last season, defenses responded by using their nickel or dime defenses on 58.3 percent of their snaps, according to Football Outsiders, a figure that surely will rise this season. And when defenses use five or more defensive backs, someone must move inside to cover the slot receiver.

"It's obviously a huge position. It's always going to be a starting position because of the importance of where it is, where they are throwing in the slot," Vikings coach Mike Zimmer said. "They probably play more than the linebacker in the base defense does. They're on the field so much with the third downs, even in the red zone on the goal line. People don't really get into goal-line [heavy personnel] that much anymore."

Defensive challenge

Surely, Spielman and Zimmer had nights like Thursday in mind when the Vikings signed Munnerlyn away from the Panthers. Munnerlyn will spend much of the game chasing around Green Bay Packers receiver Randall Cobb, who is one of the league's best at creating separation in the slot.

The Packers used three or more wide receivers on a league-high 77 percent of their snaps in 2013, according to Football Outsiders. This season, they are spreading defenses out even more often. Cobb has run a league-high 92.8 percent of his routes out of the slot, according to Pro Football Focus, and has an NFL-best five touchdowns as a slot receiver.

"He's got good, quick movements," Packers quarterback Aaron Rodgers said. "He's strong enough to get off the press. And he's catching the ball really well. He does a good job of getting separation using his release, trying to make it look the same every time and then mixing up his route running to try to get open as much as possible."

Unique spot

Cobb's potential route tree is so vast that if illustrated it would resemble a lush evergreen. But from Munnerlyn's perspective, the number of routes a wide receiver can run on the outside is so limited comparatively that it would look like the fledgling tree from "A Charlie Brown Christmas."

"Nickelback is the toughest position besides quarterback because there is so much space and there are different route concepts," said Munnerlyn, who starts at cornerback in the base defense then moves inside in the nickel and dime packages. "It's definitely different. Outside, you get the basic stuff and the sideline is your friend. Inside, man, there's a lot of space in there."

So Munnerlyn's mind-set must change when he plays nickelback. On the outside, he can afford to peek at the quarterback and be more aggressive in coverage. When covering a slot receiver who has a two-way go, he must be patient and keep his eyes on his man. If he doesn't, and the slot receiver zigs instead of zags, Munnerlyn is toast.

Besides being able to quickly recognize routes and operate in space, a good nickelback, according to Zimmer, also must be able to defend the run. Blitzing ability is a big plus, too.

"The inside is not made for everybody," Munnerlyn said.

Just ask Josh Robinson. Last season, with Winfield gone, the Vikings asked the young cornerback to replace him as their nickelback. The experiment blew up in their faces.

"Captain understands that position," said Robinson, who has thrived as an outside cornerback this season. "I told him, 'You can have it.' He does it well."

Disappointing numbers

So far, though, Munnerlyn has not been as successful with the Vikings as he adapts to a new scheme.

Despite playing 92 coverage snaps in the slot through four games, Munnerlyn has been targeted only 10 times. Unfortunately for him, nine of them have been completed for 130 yards and two touchdowns, according to Pro Football Focus. Quarterbacks have a perfect NFL passer rating when targeting him in the slot.

"He's a good nickelback," Zimmer said. "I love that he is feisty, tough, and aggressive. We just need to continue to work him within the scheme."

Munnerlyn is the first to admit he can play better, and the Vikings are confident that he will.

With the Packers looming at Lambeau Field, Thursday night would be a good time for the nickelback that Rivera first envisioned and the one the Vikings signed to show up and slow Cobb down.