Amari Cooper called it a “once-in-a-lifetime story. I never heard of anything like that until he did it.”

The Raiders’ receiver was talking about a childhood friend who didn’t play high school football and was discovered by a junior college when he was playing in the park.

That same friend who was walking off the practice field in Napa.

Undrafted free-agent Johnny Holton.

Holton turned some heads with back-to-back deep catches in Monday’s practice, just as he did in that Miami park five years ago.

“I was starting to think it was over for me and football, but I kept my faith,” Holton said. “I still dreamed about playing in the NFL one day.”

Holton is one of 11 kids, and because he didn’t take high school seriously, anyway, he got a job working at a grocery store to help his mom make ends meet.

He loved football; he and his brother had played in the park with Cooper since they were 11, and he still played in a 7-on-7 flag-football league on weekends. “Our team was called ESPN,” Holton said, smiling. “Lot of highlight plays.”

A coach from the College of DuPage in Glen Ellyn, Ill., saw Holton in the park when he was in town to see another player. He liked the highlight plays.

With the door opened, Holton quickly took and passed the GED and scored 15 touchdowns for the Chaparrals in two seasons.

He transferred to Cincinnati and showed his big-play potential, averaging 19.4 yards per catch (46 for 892) and scoring 10 touchdowns in two seasons. Holton missed seven games last year with a hamstring injury and wasn’t a coveted participant at the NFL combine.

Many teams flagged him as too skinny or too raw. The Chiefs even made him work out as a defensive back.

After the draft, Holton had offers from two teams — the Raiders and Lions — when he used a lifeline and phoned a friend.

“Of course, I gave him some advice,” Cooper said. “I want him to be successful, so I pointed him in this direction.”

Holton, who is two years older than Cooper, was leaning toward Oakland, anyway.

“I thought it would be good to team up with a friend,” Holton said. “We were never on the same team growing up.”

Cooper made the Pro Bowl in his rookie season last year, and Holton said that even at 11 years old, Cooper was a star.

“He was the real deal growing up, too,” Holton said.

Cooper said Holton wasn’t too shabby himself, and his friend already has shown the coaches his ability to beat defensive backs deep.

“He’s made some great catches here,” Cooper said. “He has qualities you look for in a receiver. No one can jam him at the line, and he’s fast. He’s developing nicely.”

Holton followed up his nice breakout with a leaping catch Wednesday that got some oohs and aahs from the fans in attendance.

Raiders head coach Jack Del Rio said he heard the cheers for Holton, but added, “We’ll learn more as we go.” That’s coach-speak for, “Don’t ask me about a guy we may try to sneak onto the practice squad.”

Making the 53-man roster will be tough, as Oakland is set at the first four receiver spots with Cooper, Michael Crabtree, Seth Roberts and Andre Holmes. Holton’s chances would improve if he returned punts well, and he got his first look there Thursday.

He will keep grinding and honor the biggest piece of advice that Cooper gave him: “Never take a day off.”

That won’t be a problem for Holton.

“I am so blessed to be wearing the Silver and Black,” Holton said. “I used to go to high school football games and just watch after I got off work. And now I am here.”

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