Sunderland AFC chairman Niall Quinn revealed the legal action had started while appearing on a BBC radio phone-in.
He was asked by a caller whether the club would allow fans to watch their heroes train, which had been allowed previously.
Quinn replied: “We had a supporter who got a bad injury. I think it was one of Djibril Cisse’s misses where he had a shot from 20 yards. I’m making fun of it now, but it knocked a supporter out. It was quite serious.
“That supporter is in the process of suing us right now.”