The referee, 46, reportedly was punched in the face after he gave the 17-year-old a "yellow card" warning for committing a foul.
There was no buildup to the incident or previous exchange of words between the referee and the player, Hoyal said, adding: "It was almost an instinctive reaction."
Initially, the referee was thought to have suffered only minor injuries. However, after the man lost consciousness, doctors at the hospital discovered he had far more serious head injuries than thought; he was then downgraded to critical condition, Hoyal said.
Granite School District spokesman Ben Horsley said the Fut Continental soccer club is listed as having reserved the field at the time of the incident. Efforts to reach a spokesman for the club were not immediately successful.
The popular Utah Youth Soccer Association does not list the Fut Continental club as one of its sanctioned member teams, which includes Taylorsville park and recreation clubs and numerous other organizations. The game was part of a "rogue league" unaffiliated with the association or any city or town recreation department, association CEO Andrew Hiatt said. The association has 50,000 youth players and 15,000 coaches statewide.
In the state association, any player or coach who puts a finger on a referee faces immediate suspension. Players and coaches also sign a code of ethics.
"It really is a sad situation. I don't think the kid went into this thinking he was going to assault somebody," said EJ Reyes, president of the Taylorsville Youth Soccer League, which is one of the 46 clubs statewide affiliated with the Utah Youth Soccer Association.
Youth players mimic what they see from parents and professional players who often fail to control their emotions and berate referees, he said.
But Reyes said he had never heard of a youth soccer player attacking a referee before, in his association or outside it.
Meanwhile, UPD is asking anyone who saw the incident, or perhaps caught it on video, to contact detectives at 801-743-5913.
Tribune reporter Michael McFall and The Associated Press contributed to this story.