Milan manager Stefano Pioli rued his side's poor play in Wednesday's 3-0 loss against Chelsea, saying the kind of mistakes made by his team "cannot be committed in the Champions League".
The scoring was opened by Chelsea's big-money signing in the middle of defense, with Wesley Fofana scrapping home a loose ball in the box following a corner in the 24th minute.
Pierre-Emerick Aubameyang and Reece James then put the game to bed in the second half with goals in the 56th and 61st minutes respectively, and they were able to restrict the visiting Italians to one shot on target for the entire game.
Pioli lamented his side's mistakes, saying teams in the Champions League will always punish you.
"Errors? It is true that they cannot be committed in the Champions League," he told reporters. "We forced choices that weren't forced. We have lost our distance. It is a pity because, beyond their qualities, we could have done something more.
"It has nothing to do with little experience, it has nothing to do with those who were not there. We had to be more lucid, in ball recovery and aggression, we had to do better.
"The second goal changed our attitude, our mentality. We weren't able to stay as a team as we usually do. At these levels, you pay dearly for these things."
He added on Sky Sports: "We had to do better. It's clear that when you make so many technical mistakes you allow opponents to put you in difficulty.
"I think the team was ready to play this type of match. There were spaces to be more precise and we gave too much field to the opponents.
"Defeats must help us grow. The locker room was rightly disappointed and angry, but this will give us great motivation and drive."
This result leaves Chelsea and Milan both on four points in Group E, trailing undefeated Salzburg (one win, two draws).
The defeat marked the sixth time in the competition that Milan have lost by three goals or more and the first time since March 2014 against Atletico Madrid.