In a blood and thunder contest at Anfield, City were starting to take command of a tight game when a VAR decision ruled out Phil Foden’s goal and seemed to reinvigorate the home side and supporters.
But though he felt Liverpool's goal was avoidable, he was pleased with his team's efforts throughout.
“We conceded a goal because of a mistake, and we didn’t follow the transition," said Pep.
"Alisson plays it out quick, Kevin didn’t follow, and Joao lost the duel - it is always so difficult against Salah with the way he uses his body.
“This is something we have to learn for future tight games against Liverpool, but the way we played, the performance and the courage we played with was fantastic.”
The City boss says he was surprised a foul was given in the build up to City’s disallowed goal – feeling it wasn’t a clear and obvious error by the on-field official.
“The referee spoke with my assistant coach Rodolfo [Borrell] and he said we are not going to make fouls until it is clear,” said Guardiola.
“At that stage (when we scored) we had all the momentum and nobody knows what would have happened after.
“In the FA Cup semi-final and the Community Shield it was so soft (the manner of defeat) and it’s a case of if they shout loud, we have to shout louder; if they run, we have to run more, otherwise in this stadium, it’s impossible.
“We handled it, we did it, but football is football - it happens.
“We had six or seven chances. They made transitions but it was a tight game - it is always a tight game against Liverpool - but at this level, you cannot concede a mistake.
“We lost because we made a mistake against one of the best teams in the world.
“They have played three Champions League finals in the past five years or so and they are an exceptional team that I admire a lot.
“The margins are tight and after they scored the goal, it was the real Anfield. Before this it was calm, but after, they were a real handful.”