Police were called to the Stadium of Light following claims Oxford United goalkeeper Jack Stevens was headbutted at half-time in the U’s 3-1 League One defeat at Sunderland.
Oxford boss Karl Robinson admitted he did not see the incident which happened in the tunnel during the break.
“We are waiting for the police to come, one of my goalkeepers was headbutted at half-time,” he told BBC Radio Oxford.
“I don’t want to use names, it’s not right to do that.”
It proved a bad-tempered game, with Robinson sent to the stands after Aidan McGeady scored Sunderland’s second, while Oxford midfielder Mark Sykes was also sent off for a second yellow card just after the hour.
Robinson added: “There was a melee, something got said, which tends to happen generally in football, Jack walked behind somebody, no malice, they just turned round grabbed him and headbutted him. Everyone has seen it, their players have seen it as well.
“This club has had more TV cameras than any other club in world football following them for the last two years and for some unknown reason they all seem to be off at half-time.
“It’s something we will deal with internally. We will be waiting for some sort of conversation to see where we go from here with this legally.”
Sunderland manager Lee Johnson suggested there was more trouble at the end of the game, but played down the severity of the half-time incident.
“Genuinely, I didn’t see what happened at half-time, I was in the toilet, waiting for the boys in the dressing room wondering where everybody was,” he told BBC Newcastle.
“Look, it happens all the time, especially in a condensed area. It was handbags a lot of it.
“It happened at the end as well, which is not ideal, and I think that one was provoked, to be honest with you.
“As a football club, I want us to act with class, but I also want us to look after each other when there’s moments.
“It was an incident that wasn’t great to see, but at the same time is probably not worth being blown out of proportion because not a lot happened.”