The other player was concussed and would be stood down for three weeks.
King said opposing team's player was "quite large" but that was not unusual in competition.
"In a lot of our grades down here there's a big difference in sizes, particularly with our country teams.
"He's one of the bigger boys running around," he said.
Nelson Marlborough Rescue Helicopter pilot Duncan Gourley said the boys were accompanied to hospital by one of their mothers.
One lived in Brightwater in Nelson and the other was from Richmond.
"From what I understand the other player was the right age, but it sounds like he was a bit bigger than the other kids," Gourley said.