"He was going to go home after training on Tuesday.
"He said to me it was personal reasons. I don't know what they were because he didn't give them to me.
"He'd made his mind up and nothing I said was going to change it.
"We had a number of discussions, but then he spoke to other people back at home and that resolved whatever problems there were.
"I know he is worried about his knee, he has treatment on it all the time. That is an ongoing thing and it will be."
McCarthy confirmed that he rang Healy, who had been on standby but that he had to shatter the youngster's dreams hours later by telling him Keane had changed his mind.
McCarthy also dismissed suggestions that Keane wanted to quit the squad following an incident with Bonner after training on Tuesday.
Keane was reportedly angry because the Republic's three goalkeepers - Shay Given, Dean Kiely and Alan Kelly - did not take part in five-a-side practice matches.
Bonner had worked the goalkeepers for 40 minutes before the rest of the World Cup squad arrived and decided to rest them - much to the consternation of the squad captain.
Aggrieved that outfield players had to stand in goal and try to save the ball with their feet, Keane clashed with coach Ian Evans, Bonner and Kelly before he was calmed by McCarthy.
Keane was first to board the team coach afterwards, but McCarthy said: "There is some suggestion this is all to do with a bust-up at training, but that is way off the mark.
"There were suggestions he had a scuffle in training, and I wasn't even aware of that - Packie has bigger arguments with his lad over his homework, it is a regular occurrence in training.
"I asked him if it was that, or anything else, but he assured me it was personal.
"The lads are the best bunch you will ever meet and they will get on with it."