Sheffield Wednesday goalkeeper Chris Kirkland was assaulted on the field during the Yorkshire derby against Leeds, which ended 1-1.
The former England international, 31, had just conceded a 77th-minute equaliser to Michael Tonge when the incident happened, with a man running onto the field from the Leppings Lane End which was housing the Leeds fans, and appearing to punch Kirkland in the face.
The man then returned to the stand while trouble continued around him, with a number of other visiting supporters running on and off the field and stewards struggled to contain them. Trouble had been brewing throughout the night, with a section of the away support throwing bottles on the field as they clashed with stewards and police.
Leeds manager Neil Warnock has called for Kirkland's assailant to be given a prison sentence as he described the events at Hillsborough as "an absolute disgrace".
Former Sheffield United boss Warnock told Sky Sports 1: "I felt embarrassed to be a manager when I saw that. I thought it was an absolute disgrace.
"I think they should get the guy and prosecute him and put him in prison.
"He (Kirkland) went down like a ton of bricks but that doesn't make a difference. Nobody should be on the pitch doing that."
Warnock added: "He spoiled everything for everyone. I've not enjoyed that when I see a moron like that.
"I am not proud of being Leeds manager when I see that. I don't mind the rivalry between us - there was a great atmosphere.
"To see things like that on the field of play there is no place for it - I am absolutely embarrassed."