I’m not sure that it’s true. Even at Barca he had been horrendously unprofessional – that was before his knees gave up. He had a spell in December where he went 5 games without scoring (seems ridiculous that he had still managed to score 34 in 37) – but in that time (a little over a month) he flew to Brazil and back 9 times. Then another 4 games without scoring in February, when he decided to visit the carnival.
Bobby Robson’s assistant, some random Portuguese chap named Jose, even openly criticized him in the press for his attitude. I won’t even talk about the fact that he had cost them the title because he decided to miss last 3 games of the season, and they’ve lost to Hercules; as this was not because of his partying but because of the whole contract negotiations/Inter transfer stuff.
From the very first night at Inter he went clubbing and only went home at 5-6 a.m. before a morning training. He had leveraged Luigi Simoni into allowing him to drink cola during and after training... except that he poured beer into cola cans. And that was before his big injuries and before he had met Vieri.
So yeah, he was never big on training, although his incredible physique not only allowed him to dominate on the pitch, but also to quickly recover after those crazy nights (Vieri had jealousy noted that while he was dying during training after their night out, Ronaldo looked like he had slept the whole night in his bed).
The only period of time when he was more or less disciplined was his stint in PSV, because his mom still lived with him. The only leeway he got was when journalists tried to get a quote from him and he traded those for beer.
He missed the last 3 games because of international duty. La Liga finished in June and Fifa refused to release him back to Barcelona