Stoops was up for the challenge, and picked out a cornerback to cover Amari Cooper.

“The five-star kid comes over, really cocky, and slaps Amari across the face,” Campbell said. “Amari didn’t retaliate, didn’t say anything, just looked at him. And then he beat the kid in the drill. One time, two times, three times …

“The fourth time, that poor DB really got burned … That was a great day.”

Campbell, who was an assistant coach on Cooper’s Northwestern High team, packs a bus and takes a bunch of high school players to football camps every summer. In 2011, he sandwiched the Florida State camp between one at Miami and one at Alabama.

“I wasn’t nervous,” Cooper said. “I just wanted to run routes and be the best route runner there.”

“The kid always carried himself as a pro, even as a kid,” Campbell said. “Quiet. Never said anything. But would always go the extra mile.”

By the time the bus went the extra miles to Alabama, Cooper was no longer the kid coming off a hip injury who didn’t have a lot of game video from his junior season.

“He was ‘That Guy,’” Campbell said. Alabama head coach Nick Saban “was all over him.”

Saban got his man, and so did the Raiders. After catching a national-best 124 passes for 1,727 yards and 16 touchdowns for the Crimson Tide last season, Cooper was the fourth pick in the NFL draft.

And he has been a blur, a silent difference-maker for Oakland.

Cooper has 24 catches for 339 yards and two touchdowns, and is third among wide receivers with 186 yards after the catch, according to Stats Inc. He joined DeSean Jackson as the only NFL rookies in the past 30 years to have two 100-yard receiving yards in their first three games.

And Cooper became the first Oakland receiver — rookie or otherwise — since Randy Moss in 2005 to have back-to-back 100-yard games.

“He’s caught some balls in the middle of the field for us,” offensive coordinator Bill Musgrave said, “and made people miss or caught balls on the sideline and stopped, started and juked. It’s amazing stuff.”

Cooper humbled Pro Bowl cornerback Joe Haden of Cleveland pretty good in Week 3, catching eight passes for 134 yards.

“He’s going to be legit,” Haden said. “His stop and start is special.”

Cooper, who’s 6-foot-1, 210 pounds, says he has been working on his footwork since an early age. He is faster than his glide makes him look — he ran a 4.35 40-yard dash at the NFL combine — and he looks like Hall of Famer Lynn Swann at times with his body control.

“I believe I have some good routes,” Cooper said in his typically understated tone.

The Raiders have been using the passing game — especially Cooper’s ability to make defenders miss after short routes — to set up the running game.

“He’s a home run waiting to happen,” quarterback Derek Carr said. “He’s a nightmare for guys to cover because he is so athletic. He knows that he can only get better from here, and he’s not going to stop working.”

Cooper has had a drop in each of the first four games, and is neither satisfied nor particularly excited about any of his accomplishments in his early NFL career.

“I really don’t feel like I have done anything yet,” Cooper said.

Cooper first “wanted to be the best ever” in the second grade. That was after the local Pee Wee coach had made a recruiting visit to the fast kid’s house for a second straight year.

“I didn’t like the hitting when I first watched, but my mom told me to think about it,” Cooper said. “I gave it a try and I liked it. Everybody in my neighborhood was pretty good, and that pushed me to want to get better fast.”

Liberty City is a rough neighborhood in Miami, and Campbell credits it with making players like Cooper and former Indianapolis running back Edgerrin James stay focused.

“There are a lot of distractions on the street,” Campbell said. “You’re trying to practice football and there are some kids hanging over the fence counting their money. You have to have something inside of you to block that out and focus.”

Cooper credits his mom, Michelle Green, with his work ethic.

“I saw how hard she worked,” Cooper said. “She influenced me a lot.”

Green didn’t know how much until she heard Cooper say it moments before the Heisman Trophy ceremony last year. Cooper, who finished third, told ESPN about a mistake he made when he was 10.

His mom had come home from working two jobs and asked him to go with her on a 3-mile walk to the grocery store. Cooper said he was too tired, then felt terrible later when his mom came home with marks on her arms from having carried all the bags herself.

“She did everything for us,” Cooper said, noting that besides her three kids, Green also helped raise two nephews. “She sacrificed for us, and I was selfish that night. That stuck with me.”

So Cooper practices, watches game video, runs routes on his own and practices some more. And he often asks teammate Michael Crabtree for tips on how to use his hands better to get past cornerbacks off the line of scrimmage.

“I prepare as if I want to be a great player, and I believe anytime you do that, you will be successful,” Cooper said.

An early-season favorite for Rookie of the Year honors, Cooper has bigger goals in mind. But, surprise, he says he keeps those to himself.

“I will be very surprised if he is not a Hall of Fame player,” Campbell said. “He works very hard to be the best. A lot of kids like football, but Amari loves football. That’s his whole life. He has nothing else going on.

“I knew he was special right off the bat.”

Vic Tafur is a San Francisco Chronicle staff writer. E-mail: vtafur@sfchronicle.com Twitter: @VicTafur