New D.C. United midfielder Luciano Acosta, second from right, “is electric and something different, a special talent,” Coach Ben Olsen said. (Toni L. Sandys/The Washington Post)

Luciano Acosta was 16 and pursuing a pro soccer career in Argentina when his agent took him to see a doctor. With the physical demands of the game growing, they wanted to know if he would too.

“You are not going to grow anymore,” D.C. United’s winter acquisition remembered the doctor telling him. “This is it.”

He stands 159 centimeters, or 62.6 inches — not quite 5 feet 3. Even by the welcoming standards of soccer , in which height disqualifies only aspiring goalkeepers and central defenders, Acosta was undersized.

What he lacked in stature, however, he compensated in technical skill and quickness. His feet were hummingbird-fast. He dodged challenges and accelerated into space. He kept the ball close, darting from danger and releasing killer passes.

“If I couldn’t achieve it with height, I could do it with my feet and other things,” the 21-year-old said through an interpreter ahead of United’s MLS opener at Los Angeles on Sunday night. “I did not need size. People doubted me. I wanted to prove they were wrong.”

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Acosta’s subsequent development as a player lifted him to the height of Argentine soccer: a first-team contract with Boca Juniors, Latin America’s most revered club.

After just under a year with Boca Juniors, the team he adored as a boy, Acosta went on loan to a league foe, Estudiantes. Last month, Boca was set to loan the midfielder to Brazil’s Coritiba when United came calling.

For two years, Coach Ben Olsen had squeezed everything he could out of a gritty squad. What he coveted was a dynamic central attacker with the capacity to make improvisational runs, sustain possession and execute clever combinations with a capable cast of returning players.

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Acosta arrived on a one-year loan from Boca late in preseason, in a deal that includes an option for United to purchase him outright at the end of the year. The price is $2 million, steep for a thrifty MLS club, but such fees are typically negotiable.

In his first serious match, the first leg of the CONCACAF Champions League quarterfinals against Mexico’s Queretaro last week, Acosta exhibited the skill set for which United has yearned.

“He is electric and something different, a special talent,” Olsen said.

Despite a sterling debut, Acosta did not start the return leg against Queretaro on Tuesday. Needing to move his girlfriend and 14-month-old son to Washington, he had flown to Buenos Aires after the game in Mexico and did not rejoin his new club until two days before the decisive rematch at RFK Stadium. He entered early in the second half of the eventual 1-1 draw (3-1 aggregate defeat).

United wants to temper expectations. Acosta is, after all, young, playing abroad for the first time and adapting to a new culture, league and team. “This is going to be a big transition for him,” Olsen said.

Acosta’s teammates have eased it. Brazilian midfielder Marcelo Sarvas, an MLS veteran who is also new to the club, has taken him under his wing. Acosta does not speak English; Sarvas is fluent in English, Portuguese and Spanish.

With a 13-year age difference, Acosta said, “Marcelo says he is like my dad.”

He has also bonded with forward Fabian Espindola, a fellow Argentine and Boca Juniors alum, who said he sees the Boca influence in the way Acosta practices and carries himself. Like many aspiring Argentines, Acosta arrived at Boca’s academy center at age 13.

“The way they treat you when you are young, it’s very tough,” Espindola said. “Even though you are a kid, it’s like you are professional already. If you play two games bad, they’ve got another kid ready to go. There is no room for error.

“You can see he is ready to go. When he came here, it was: ‘Damn, he’s good.’ ”

Acosta is from the Lugano neighborhood in southern Buenos Aires. His father, Juan, has worked as an electrician and bus driver. His mother, Sara, looked after six children.

In a city divided by soccer allegiance, Juan supported River Plate while Sara backed Boca. Luciano sided with his mom because, he said, he and his dad were always competitive. The kids were split as well.

“We screamed at each other, during the game, during the goals,” he said. “It was a fight in the family.”

Acosta was 8 or 9 when he first played organized soccer for the Miraflores youth club. By 13, he was in the Boca system. Boca uses academy players as ball boys, so at 15, Acosta was running the sidelines of the club’s famous stadium, La Bombonera (“The Chocolate Box”).

Amid a festival of sights and sounds, he made his first-team debut in February 2014. “The first time I played before the fans,” Acosta said, “it was beautiful.”

Over parts of two seasons there, Acosta made 30 appearances across all competitions. In his first match against River Plate, in a severe rivalry known as the “Superclásico,” Acosta’s effervescent performance earned him a nickname from a journalist: La Joya (“The Jewel”).

The competition for playing time at Boca was intense, so the club loaned him to Estudiantes, where last year he appeared in 27 league matches (11 starts) and seven games in Copa Libertadores, the continental competition.

In scouting Acosta, United was not turned off by his size; aside from the height, he weighs 132 pounds.

“It’s a great sport, isn’t it?” Olsen said. “It doesn’t enter my mind, especially with attacking players. As long as he is quick and has special qualities, I don’t care if he’s shorter in stature. Is he equipped to be a center back? Nah. Fortunately, he’s not a center back.”

Last year’s MLS MVP, Italy’s Sebastian Giovinco, is 5-4. The world’s best, Lionel Messi, is 5-7.

“It’s always a fight, no matter how tall you are,” Acosta said. “You still have to be able to play.”