The home side were the better side for most of the first-half with Liel Abada going close twice and Callum McGregor hitting the woodwork.
Daizen Maeda fluffed his lines after the break before the visitors took control with goals from Vinicius, Luka Modric and Eden Hazard.
“I wasn’t angry,” Ancelotti said, regarding the first-half performance. “I can’t get angry with this team. It is true that we struggled but in the end we got there.
“Second half we did play well. Every team can struggle including us but in the end we kept our heads and managed to stay in the game.
“Celtic is a good team. We were worried about the game because we knew what they did against Rangers. We suffered in the first half but it was normal to suffer here. The atmosphere at the stadium is fantastic. We are happy to show good football to these supporters.”
Ancelotti believes the experience of playing at Celtic Park will be of huge benefit to the younger players in his squad. While Toni Kroos, Luka Modric and Karim Benzema are all used to such environments, the Spanish giants had a number of individuals 24 and under.
“Of course young ones don’t have the experience of the older ones but they are used to handling the situation and the atmosphere,” he said.
"They like to play football and to play in this kind of atmosphere and this kind of stadium. It is a good experience.”