LOS ANGELES — It was a behind-the-scenes look at how a player like Jalen Brunson, without the requisite size or athleticism, can become an NBA star.

The viral video, discovered by X account @NBA_NewYork and viewed over 4.5 million times as of Monday afternoon, shows a pre-teen Brunson, exhausted and exasperated, sludging through a one-on-none shooting drill while his father is relentlessly pushing focus and discipline.

“Tired is for the weak,” Rick Brunson, the dad, says in the video.

“PTSD,” Jalen responded Monday.

The workout, filmed by Brunson’s mother, Sandra, took place between 2007-09, the same time Rick was an assistant coach at the University of Virginia.

Brunson said Monday his annoyance was about having to train outside in the “hot-ass sun,” rather than inside the air-conditioned gym at UVA.

A young Jalen Brunson shoots a jumper during a workout with his dad. PlayersTV Shorts

But it was clear from the mood of the video, Brunson was being groomed to disassociate himself from excuses.

“You probably saw how pissed I was at the moment,” Brunson said. “But I guess it all worked out.”

What the video didn’t explain is why Brunson, a natural righty, is shooting with his left hand.

That was also because of Rick.

And it remains that way.

As far back as his days practicing on a Little Tikes plastic hoop, Brunson was told to keep the ball in his left hand.

He can’t remember ever playing otherwise.

It was a maneuver by Rick to boost his son’s advantage in basketball since lefties are more difficult to guard, just because they’re unorthodox.

“I do everything right-handed. Writing. Eating. All that stuff,” Brunson told The Post. “But I didn’t know that he basically made me keep the ball in my left hand until a certain age. So naturally I shot the ball lefty. I didn’t know it was a planned thing that he did when I was younger.

A young Jalen Brunson shoots a jumper during a workout with his dad. PlayersTV Shorts

Rick and Jalen Brunson Charles Wenzelberg / New York Post

“It got to the point I was so comfortable, I didn’t know it was a thing.”

Today, Brunson is part of the roughly 10 percent of NBA players who shoot with their left hand, which the 27-year-old used last week to bury all nine of his 3-pointers and score a career-high 50 points against Phoenix.

His father, a lefty himself and former Knicks point guard, is now on the Knicks bench as an assistant coach.

“His message [to me] is the same, but I guess the approach is different,” Brunson said. “He was trying to make me mentally tough and stuff like that. It’s pretty much the same. In a good way.”

Knicks guard Jalen Brunson #11 goes up for a shot as Detroit Pistons guard Marcus Sasser #25 is too late to defend during the third quarter on Nov. 30. Charles Wenzelberg / New York Post