Lopetegui insists their performance deserved much more on the day.
He stated, "It's tough for me to explain the score.
"It's true that we did a lot of good things. In my opinion, in the first half, we overcame a lot of things, but we didn't take advantage of our good things and positive things.
"In football, if you don't take advantage then normally, against this kind of team, you are going to be closer to suffering a goal (being scored against you).
"But the first goal was very, very unlucky. It was not about one moment when they dominated us or they had clear chances. It was one moment when it was one rebound (off Coufal’s boot into the net) and it was very unlucky for us.
"It was one key moment, above all, and it came after we had a very clear situation to score, which we didn't do. And when this happens, normally you are going to lose the match.
"There was another key moment when Crysencio Summerville regained the ball in a very clear press and the referee whistled for a foul, but it was not at foul. It was very clear and had a very good press. It was a very clear chance for us, a one-on-one with the ‘keeper.
"So, when all the key moments work against you, against this kind of opponent, normally you are going to lose, but that's why it's not easy for me to explain the match.
"We had double the number of shots that Manchester City had, and that's why it's not easy to explain the score.
"We have to keep the positive things, knowing that the little details we have to improve. We have to improve to be more consistent in both boxes and in the moment when you don't take advantage and score yourself, you have to avoid above all conceding the second and third goals.
"The last City goal was one moment where we had a very clear ball and we can change and switch the play but we didn't do it. But they are goals that we can set.
"There were no goals when they dominated us and they had very clear chances.
"That's why in these little details and individual actions we can and we have to do better."