The charge is less serious than manslaughter. It carries a possible sentence of up to five years in prison for adults, but penalties can be less for juveniles.
Gill said it became clear in looking at the facts that the teenager's actions didn't amount to murder or manslaughter.
"We did not believe we could demonstrate the premeditation or intent to justify those charges," Gill told The Associated Press. "Those other charges require another type of mental state. We did not believe that type of mental state was present."
The oldest of Portillo's three daughters, Johana Portillo, declined comment about the charges through family spokesman Tony Yapias.
She just wants to let the criminal justice system to run its course, Yapias said.
Johana Portillo, 26, previously said the charge doesn't matter because it won't bring back her father.
Friends and family spent Wednesday remembering Ricardo Portillo at an afternoon wake that was to be followed by an evening religious service.
The 17-year-old has been in juvenile detention since April 27 when the incident occurred at a recreational-league soccer match in a Salt Lake City suburb.
The teenager, whose name is being withheld by The Associated Press because he's a minor, turns 18 in October. His first court appearance has not yet been scheduled.
His attorney, Monica Diaz Greene, was not immediately available for comment.
The teenager was playing goalie when Portillo called a penalty on him for pushing an opposing player. The teenager began arguing with Portillo and then struck him in the head, authorities said.
Charging documents show witnesses said the teenager threw one punch, hitting Portillo in the rear jaw area with a closed fist. Another witness said Portillo was writing notes on his notepad when he was hit.
Portillo was taken to a hospital in fair condition. Hours later, he went into a coma. He never regained consciousness and died Saturday.
Police say hospital staff members told them Portillo had hemorrhaging and a traumatic brain injury. The medical examiner's autopsy concluded he died from injuries related to the blow to the head. The death was ruled a homicide.
The fact that the teenager will soon turn 18, combined with the seriousness of what happened, led Gill to push to have him tried as an adult. The chance of the teen's rehabilitation is minimal, prosecutors said in court documents. A juvenile court judge will ultimately rule on that request.
Little is known about the teenager. His coach, James Yapias, has said he was playing his first game with the team but declined to elaborate any further about the young man or his family. League president, Mario Vasquez, said he didn't know the teenager.
Tony Yapias said Wednesday he knows people want to know more about the teenager, but urged them to allow the criminal justice system to run its course.
"We cannot bring Ricardo back no matter what we try to do," Tony Yapias said. "It's a sad day. We have to reflect on this — we all have a passion for sports. Hopefully this will be something we all learn from."
Funeral services were being held Wednesday in Salt Lake City for Portillo, who leaves behind three daughters and four grandchildren. At a wake in a community center, a closed wooden casket was topped with large white flowers. Two other flower arrangements flanked the casket, with a large framed picture nearby of him wearing a blue shirt, his arms raised in victory.
The attendees wore white T-shirts with a drawing of an orange referee jersey in the middle with the words, "In loving memory of Ricky" around a soccer ball.
After the funeral, Portillo's remains will be returned to his hometown of Guadalajara, Mexico. He moved to Utah from there 17 years ago.
Family members said Portillo loved soccer, and had been working as a referee in different leagues for eight years. His oldest daughter, Johana Portillo, said he had been attacked before by players, even having his legs and ribs broken. Though his daughters begged him to quit, he said he couldn't give up his passion.
Portillo was a longtime referee. But he was not registered by the United States Soccer Federation or trained, evaluated or supervised by the Utah Soccer Referee Committee, which oversees those who referee games played by the Utah Youth Soccer Association.
The incident occurred in a game held by La Liga Continental de Futbol, an unaffiliated soccer league created in 2009.
Tony Yapias said Ricardo Portillo was well known in the community, and once said that he would someday be a famous soccer referee.
"How ironic," he said. "What has happened as a result of this is just that."