'I knew that was bad so I stood up to him, this idol of Manchester, this great guy who everyone loved, and replied: ‘f*** off, you'
The former Argentina defender, who is now manager of Argentinos Juniors, played alongside iconic midfielder Keane for the Red Devils between 2004 and 2007.
The 43-year-old, who retired after spells with Real Madrid, Marseille and Roma, revealed that the former United captain knocked him out during a row following a game.
Heinze told ESPN Argentina's Pura Quimica in 2017: “We lost a game and I went in the dressing room first and Roy Keane was second. I liked to go first after the game.
"I didn’t want to speak to anyone as we lost. I didn’t understand English, just the bad words. I heard my name and 'f*** off' by Roy Keane, the best player.
“I knew that was bad so I stood up to him, this idol of Manchester, this great guy who everyone loved, and replied: ‘f*** off, you.' I don’t remember what happened next."
However, Heinze was prepared to apologise after receiving a phone call from legendary boss Sir Alex Ferguson the next day.
He added: "Ferguson called me up and told me that he knew what had happened. I asked him if I needed to apologise... I mean he was the biggest idol of Manchester United's history.
"He said, 'no, Roy was like that.' But he told me not to worry because Roy had already forgotten about that episode. We went to training and I was very scared.
"I remember we both went for a ball and I was ready for anything but as the action happened Roy told me, 'what happens on the pitch stays on the pitch and what happens in the changing room ends there.'"
Heinze said he was relieved then that they went on to have a good relationship off the pitch.
He said: "I've had an excellent relationship with him since that episode.
"He is an extraordinary man. He was one of the best captains the club ever had."
Despite that incident, Keane, in his second autobiography, spoke well of Heinze, saying he liked him as a person.
"Gabriel Heinze was another good guy," the Irishman wrote, before going on to say how the defender once left him with a debilitating injury.
"He was a nasty f***** – nasty in training. I picked up an injury one day. A lot of it was my own fault. It was a Friday. We were playing Spurs at home the next day, and we'd always have light training the day before a home game.
"But it got a bit nasty and it ended up with myself and Gabby having a few tackles on each other.
"He kneed me on the side of my leg and, being the hero that I am, I kept my mouth shut. I didn't want to go in for treatment, but I was in agony. I left the house the next day, limping.
"I said to my wife, 'Well, I won't be able to play. I'll just go in and tell them'. I got to Old Trafford and limped – literally – to the dressing room. I got a few painkillers, and played.
"My wife said it was the funniest thing, hearing my name being announced on the radio, after she’d seen me limping out of the house that morning. But I liked Heinze."