Aaron Burbridge entered Michigan State as a top-100 recruit, and his promising freshman season suggested he’d soon fully realize his potential: Before his sophomore campaign, he was on the watch list for the Biletnikoff Award, which is given to the nation’s top wide receiver.

Instead of taking the next step, however, Burbridge dropped off the radar.

What happened?

On Saturday, the 49ers’ rookie candidly fielded that question. He didn’t work hard enough.

“I was just out there running around thinking I could still do what I did in high school,” Burbridge said. “Not knowing the game. Not knowing coverage. Not knowing when a corner jumped outside, how to break off my route a different way. Just having one way to run a route. I had to get my football IQ up and everything.”

Fortunately for Burbridge, he went from disappointment to dominant just in time. As a senior, Burbridge was named the Big 10 Receiver of the Year after he had more receptions (85), receiving yards (1,258) and touchdown catches (seven) than he had managed in his first three seasons combined.

In June, Raiders rookie quarterback Connor Cook, MSU’s starter during Burbridge’s last three seasons, said a deadline spurred action.

“I think confidence played a big part and just him knowing it was his last shot last summer,” Cook said. “He knew, ‘Hey, I’ve got one more year of college left and I’ve got to do it big.’ I think those were the main two elements. It was like, ‘I’ve got to have a bang-up year. This is my last year, and I’ve got to give it everything I’ve got.’ And once he started to get a little bit of confidence, he just took off.”

Despite finally soaring, Burbridge tumbled to the sixth round of this year’s draft, partly because of his otherwise pedestrian career. In draft circles, he’s known as a one-year wonder, a prospect whose inconsistent production can raise questions about focus and commitment.

For his part, Burbridge says his humbling college experience made an indelible impression. During his career, he lost his starting spot to Tony Lippett, the Big 10 Receiver of the Year in 2014, based on a 65-catch, 1,198-yard season. Burbridge, who had 29 catches for 358 yards, saw many of Lippett’s exploits from the sideline.

“I saw him making plays, but there was a reason I had been starting in front of him,” Burbridge said. “I could do the same things he was doing, but I saw how he went about things in meetings and practice. He practiced hard. He studied. He played with confidence.”

In May, Burbridge’s draft fall came with a silver lining: He landed in Santa Clara, where he’s one of many unproven wideouts.

Of the 12 wide receivers on the 90-man roster, nine have fewer than 14 career catches. The most proven pass-catchers, Torrey Smith, 27, and Jerome Simpson, 30, are the only wideouts over 25.

“There’s opportunity here,” Burbridge said. “Torrey and Jerome are our veteran guys, and I’m just trying to pick their brains.”

In addition to knowledge, Burbridge (6-foot-1, 208 pounds) hopes to gain a reputation for having reliable hands. He struggled with drops in college, which could be partly due to his 8¼-inch hands, which matched the smallest among wideouts at the combine.

As a sixth-round pick, he’s not guaranteed a spot on the 53-man roster, but he can guarantee this: If he doesn’t survive final cuts, it won’t be because of lack of effort.

“Sometimes I think about what I could have done (in college), but it was part of a learning experience,” Burbridge said. “You have to study. You have to watch film. And it has to be done every day.”

Eric Branch is a San Francisco Chronicle staff writer. Email: ebranch@sfchronicle.com Twitter: @Eric_Branch