"We haven't done it earlier because we didn't want people to be overconfident and think that nothing will happen if the balls hits them," he added.
Participants are given safety instructions, and warned against taking a direct hit.
"If there is no escape, it's better to lie down and let the ball run over you," the runners are told.
Witnesses said the injured man "froze" as the ball approached him at the end of the route, crushing him against a safety barrier.
"People who are used to bulls know that they can't just stand still in front of them because they could be gored," de los Nietos said.
"But with the ball some people think that nothing will happen to them, but this is not the case. If they get a direct hit, it can be serious."
He added that another option being considered was to pad the barriers, erected to keep the ball rolling along the route.
Mataelpino changed bulls for white balls on its 550m run eight years ago after locals had a brain-storming session in the village bar.
"Between jokes and bottles of beer, someone said why not roll a stone down the mountain like in Indiana Jones," said José Luis Russel.
This year 1000 people took part in the Boloencierro (ball running) and dozens of children participated in a separate event using smaller balls.
"We were expecting more people this year because of last year's incidents. People love the thrill," said Carlos Hernández of the Boloencierro Association.
