Patrick Cronin, 19, died in hospital shortly before 8.30pm on Sunday night, according to a statement from Victoria Police.

The Eltham teen - who had never been in a fight before Saturday night according to a family friend - suffered a critical head injury outside The Windy Mile in Diamond Creek on Main Road at 11pm on Saturday.

Mr Cronin played football with the Lower Plenty Football Club and its president Grant Barden, a family friend, said the teenager was trying to help out a friend.

"Patrick's never been in a fight in his life. He's not that kind of kid," Mr Barden said.

"He's tried to drag his friend out of the fight, but then someone's come up behind him. We heard he was king-hit. That's all we know."

The teen and his friends then walked back to a house on Happy Valley Rise after the incident.

Paramedics were called at 1.18am on Sunday, about two hours after the fight, and found the teenager had suffered a head injury, an Ambulance Victoria spokesman said.

He was taken to the Royal Melbourne Hospital in a critical condition. He was later taken off life support.

Homicide Squad detectives are investigating the circumstances and how Patrick sustained the injury.

A woman at the house where Mr Cronin collapsed said she was asleep when he came home with friends. She did not want to be identified.

The incident shook parents and supporters on Sunday as they watched the Research Junior Football Club play at the Lower Plenty oval in Montmorency.

The team flag was flown at half-mast to show support.

Mr Cronin had previously played for the junior team, the Searchers, and spent the past three years with the Lower Plenty Bears, an affiliated club.

On Saturday, he had just played his first match with his older brother, Lucas. It was a moment promoted on the team's Facebook page.

"The Bears didn't come away with a win, but it did finish with a couple of really proud parents," the team stated.

The club's president, Mr Barden, tried to comfort those who came down to watch the game on Sunday after hearing about what happened to Mr Cronin.

"We're all absolutely shattered," he said. "He's a really good kid. A really good student."

Mr Barden said the teenager came from a "beautiful family". His father, Matt, serves as vice-president, while his mother, Robyn, organises club merchandise.

"We're all thinking of them. It's devastating," he said.

The Research Junior Football Club also notified its followers about the incident and shared a message from Mr Cronin's mother: "Please pray for our beautiful boy, Pat, who was cowardly assaulted last night."

The Windy Mile did not return phone calls made on Sunday, but staff posted on its Facebook page: "The Windy Mile Team are devastated - our heartfelt thoughts and prayers go out to Patrick and his family and friends at this very difficult time. In light of these circumstances we will remain closed today and tomorrow."

Anyone with information is urged to call Crime Stoppers at 1800 333 000.