So, think you can beat him?
Two or three nights a week, Johnson and seven or eight Arizona players hunker down at a teammate's house to unwind with some video games. Sometimes, Johnson said, he'll play online, where anyone with a game console can challenge him in one of his favorites, the basketball game NBA 2K15.
Johnson could be playing in the real NBA this season, if not for age restrictions. He has settled for the pixilated version.
The sport's administrators don't know what to make of Johnson and other assumed one-and-done players who flash across college basketball each season like bolts of lightning.
Kentucky's Karl-Anthony Towns, Duke's Jahlil Okafor and Ohio State's D'Angelo Russell are among the most exciting players in a sport some see as lagging. But if they leave after just a few months, are they worth it?
NBA Commissioner Adam Silver has called extending the NBA's age minimum from 19 to 20 a top priority.
"That's absurd," Johnson said in a recent interview. "Let people talk. Whatever man. You can try your rules. You're not going to make freshmen ineligible in the Pac-12. It would kill the sport. The really, really good freshmen would probably start going overseas, in my opinion."
Imagine this season without Russell or Okafor, Johnson said.
"These are two of the best players in college basketball, and you're taking them right out of the sport just because they're freshmen?" he said. "It doesn't make any sense to me."
Johnson doesn't know if he would have gone to college if there were no age restriction, which has been in place since 2006. But he has called his time at Arizona essential, even if ends up being for one year.
Johnson is projected as a lottery pick if he makes himself available for the next NBA draft, meaning he could land with his favorite team, the Lakers.
Even before conference season began, Arizona Coach Sean Miller spoke of maximizing Johnson's "one-year window." So catch him while you can. Arizona plays Xavier in the West Regional semifinal on Thursday at Staples Center.
In a basketball sense, Johnson has "grown at a microwave rate," said the analyst and former NBA player Kenny Smith, who has been Johnson's mentor since Johnson was in second grade.
Johnson averages a team-high 14 points, and 6.6 rebounds, per game. Arizona guard T.J. McConnell said Johnson is the most talented teammate he has ever had.
"He has the tools," Smith said. "He's the farmer that has the rake, the hoe, the plant seed, he's got everything."
Johnson has been preparing for a life in basketball for years. At 12, he carved up defenses with a killer right hand.
His mother then gave him an ultimatum: "You're not going to play a game or practice until you can dribble or make left-hand layups," she said.
So Johnson practiced using only his left hand. He wrote using his left hand. He ate and brushed his teeth using his left hand.
It took two months, but the hand developed. By the end of high school, his coach at Santa Ana Mater Dei, Gary McKnight, said Johnson received so much attention that he started using two phones.
Johnson chose Arizona over USC because it was close and warm, and he could win there. No player likes to lose. Johnson repels it.
At Mater Dei, Johnson won four state championships in four years, a first in California basketball.
When Johnson first arrived in Tucson, he said, Miller was rough on him. Johnson was selected as the most valuable player of the Maui Invitational tournament in November, but for a stretch he seemed to be playing for scouts, not for Arizona.
He would do things Miller's way, the coach said, or he wouldn't play.
So Johnson learned.
Parts of his freshman year have been uncomfortable. Everything is new and overwhelming. There is value in that, Johnson said.
"It makes you an adult, in the simplest sense," he said. "If I went from high school to the NBA, not only would I not know what to do with myself, I would have $3 million, a bunch of money to blow. So for me, I'm so happy I came to college to learn about myself. I wasn't ready to live by myself."
Take his living situation, for example. Johnson lives off campus, so he has to be responsible for everything: rent, bills, buying furniture. The Xbox too. He's still a college student, after all, with time for the occasional online game.
Anyone can challenge him.
"If they want to," Johnson said. "I'm not sure if they want to lose."