Kuba Moczyk, 22, was knocked out in the third round of Saturday's fight at the Tower Complex in Great Yarmouth.
He is on a life support machine after being hit on the right temple, according to the Eastern Daily Press.
Mr Moczyk's trainer, Scott Osinski, described him as a "lovely lad" and "gentle" boxer.
"He never wanted to hurt anyone," he said. "He just wanted to show people that he had talent as a boxer."
Mr Moczyk's twin sister, Magdalena, is fundraising to try to get specialist help for her brother.
She has been by his bedside since he was taken to Norfolk's James Paget Hospital.
"We need to get him to a specialist hospital abroad where they can help wake him up and help him recover fully. This is our biggest hope," she said.
Mr Osinski said Mr Moczyk was winning the fight when he took the fatal blow.
He confirmed a medical team was ringside at the time and attended to the boxer before paramedics arrived.
Great Yarmouth Borough Council confirmed the venue had a premises licence to hold boxing events.
However, a spokesman said environmental health officers would be investigating as concerns had been raised over health and safety.
The promoter of the fight, Aurelijus Keree, said he was "deeply saddened" by what had happened but did not want to comment further.
Local amateur boxing promoter Leon Docwra, who trained Mr Moczyk three years ago, is calling for more governance for unlicensed boxing, which is not illegal.
"A governing body is needed to regulate unlicensed fights to make them safer," said Mr Docwra.
Norfolk police said they were aware of the situation but "would not be investigating the matter".
An amateur boxer is in a coma after he suffered a severe head injury in his first bout.