The 14-year-old was attacked when the man invaded the pitch by climbing over a rope barrier during an under-15s match.
It is believed the man also took a swing at another boy before parents of players from both teams removed him from the field.
Police have launched an investigation into the incident in Salford. The boy was taken to hospital by his father after the match.
The incident happened during a match between De La Salle FC and Radcliffe Town in the Tameside Football League under-15s match yesterday.
It is believed there were about 10 minutes to go in the game at De La Salle's Lancaster Road ground when two boys got into a scuffle near one of the corner flags.
Other players ran over and then one of the dads from Radcliffe Town ran on the pitch and punched one of the De La Salle players before taking a swing at another.
One spectator, who asked not to be named, told the Manchester Evening News: "The players are just kids at the end of the day. It's just really sad to see something like this."
De La Salle won the match 3-2. It is believed a complaint was made to police after the final whistle.
The incident is due to be discussed by De La Salle committee members at a meeting tonight.
The referee is also due to submit a report into the incident to the league which will then be investigated by the Football Association.
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