Gianluca Busio used to hold bragging rights over Joe Scally.

Busio, at 15 years and 89 days, became the second-youngest player ever to sign an MLS contract this past summer when he inked a deal with Sporting Kansas City. Only Freddy Adu, who signed his historic contract as a 14-year-old with DC United in 2003, had him beat.

“He would always brag to us that he was the second-youngest [to sign] when we went to national team camp,” Scally joked about his friend Busio to The Post.

“Now, I beat him.”

At 15 years and 80 days, Scally became the new second-youngest signing in MLS history — beating Busio by nine days — when he inked a pro contract with New York City FC on Wednesday, making him the club’s second academy player to do so.

The native of Lake Grove, NY, joined NYCFC’s academy two years ago as a central midfielder before transitioning to right back, where he instantly excelled and became one of the club’s most promising prospects. That progression, according to Sporting Director Claudio Reyna, made it a simple choice to have him follow in the footsteps of James Sands, who signed his deal as a 16-year-old last summer.

“We have Anton [Tinnerholm] and Saad [Abdul-Salaam] and then Joe is our next one in our depth chart,” Reyna told The Post. “We want to get to a place where in the next two to three years, he can work his way up the depth chart and now’s a good time to bring him in. … He was always one of our top players in the academy that we were watching and monitoring the situation.”

Together with Sands, whom Scally considers a close friend, NYCFC’s homegrown players could become mainstays in the defensive third for years to come. He and Sands will be on the same path, which includes games with the Under-19 academy squad with the hope Scally will be included in the United States’ Under-17 men’s national team, an age group he featured with at the Nike International Friendlies in December.

Scally was one of five academy players who joined NYCFC for the preseason this year, when he gained valuable experience playing alongside professionals in friendlies against LA Galaxy and Los Angeles FC of MLS and Mexican side Atletico San Luis. He stayed close to defenders Sebastien Ibeagha and Maxime Chanot, seizing on tidbits such as Chanot’s advice to always look for runners on the back post when crossing the ball in the box.

“[Scally] really fits the profile of outside back that we want at the club,” Reyna said. “He can cover lots of ground, he defends well, he gets forward well. … He’s improved so much on both sides of the ball in the two years of the academy.”

Scally came away somewhat starstruck from playing next to NYCFC’s big names and being coached by Patrick Vieira, whose playing highlights Scally admits he’s watched on YouTube. In the preseason, Vieira and Reyna were able to get a glimpse of Scally up close, and came away feeling he looked like a regular.

“It was unbelievable,” Scally said. “I wasn’t intimidated because they took me under their wing and treated me like them right off the bat.”

For now, Scally knows his biggest adjustment will be transitioning from a ninth grader into a professional soccer player. He isn’t a fan of the Red Bulls (“I tried out for [them] and they cut me”) and he plans on putting his first check straight into the bank. The plan is for Scally to finish the school year at Sachem North High School, where he’ll continue his 15-hour days from the time schools starts until the time he gets home from academy practice (10 p.m.), before training full-time with the senior team this summer.

On top of all that, what is he looking forward to most when he’s called into the 18?

“Just playing alongside David [Villa] is awesome,” Scally said.