Rio Ferdinand revealed Roy Keane once punched Manchester United team-mate Chris Eagles square in the face during a Red Devils training session.
Former Irish international Keane was famed for his aggressive nature and will to win during his 12-year spell at Old Trafford where he won seven Premier League titles and the 1999 Champions League trophy.
United legend Ferdinand recalled the moment Eagles, who played 17 games over five seasons at the club, provoked Keane into a violent assault during a reserve team training session.
Ferdinand told the VIBE with FIVE podcast: 'I remember when Roy Keane punched up Chris Eagles one time. He (Eagles) was only young kid, (taking) liberties. It was good for him Eagles.
'Roy must have been coming back from an injury, or he didn't play, and he was playing with the reserves and the youngsters were in there like Eagles and Lee Martin were like rats, getting in close and trying to tackle.
'He went in late on Roy once or twice and Roy said to him, "Listen, slow down". The ball comes in again, Eagles was trying to impress, trying to leave him mark and left a little bit on Roy again and pulled his jersey.
'Roy just turned round and went bang in the jaw. A quick short jab and I just stood there and went, "Roy, what's wrong with you?".
'And I went inside and said, "Roy, why are you doing that for? He's a young kid man. And I warned him.'
Eagles came through the United youth academy in 2003 but never got a consistent run in the first-team under Sir Alex Ferguson.
He spent two loan spells at Watford and other temporary stints at Sheffield Wednesday and NEC Nijmegen before joining Burnley.
Eagles, now 35, would go on to play in just two Premier League campaigns with his most recent club being Oldham Athletic in League Two.