With the game 0-0 at half time, Roberto Lewandowski netted twice in as many minutes before Lamine Yamal broke yet another record and Raphinha put the icing on the cake with a fourth.
Ancelotti spoke on his forward after the game, explaining he knew that Barcelona would try to catch him offisde.
A defiant Carlo Ancelotti (pictured) has given his verdict on Kylian Mbappe's performance against Barcelona
'It was known that they use the high line and we almost didn't take advantage of it,' he said. 'He had chances and sometimes he was offside.
'But we had three or four opportunities where they needed more success. I don't regret our approach. We did have opportunities.
'It's a tough moment, as always when you lose. And even more so when you do it after so many games unbeaten.'
After Raphinha's goal, meanwhile, it appeared to get heated on the sidelines as the two benches clashed.
Ancelotti and Barca boss Hansi Flick both appeared to be involved, and spoke on the incident after the game.
'I haven't seen it,' Flick said. 'I've spoken to him, but it's a normal situation. We celebrated the goal. We're adults enough to talk about this.'
Ancelotti, meanwhile, added: 'I didn't argue with Hansi Flick, I got angry with one of his staff. He made gestures towards our bench, he didn't act like a gentleman.'
It is a result that puts Barca six points clear at the top of LaLiga, leaving more questions than answers for a Madrid team which was expected to evolve into almost world beaters this season.
But manager Ancelotti isn't worried, defending his team after the game and appearing to identify the result as almost an anomaly on the back of the humiliation.
'We don't have to throw everything in the trash,' he said. 'We have to forget the last 30 minutes.
'It does not reflect what happened on the field. We couldn't get ahead on the scoreboard and they took advantage. The game, until the first goal, was very even and we had more chances.
'We already lost 4-0 here to Barca and won LaLiga and the Champions League. We won't be far from last season.'