"Forty five minutes," Guardiola said when asked what had gone wrong after the game.
Manchester City manager Pep Guardiola cited a poor first half performance as the reason for his side's Champions League round of 16 exit at the hands of AS Monaco.
"Forty five minutes," Guardiola told BT Sport when asked what had gone wrong.
"We played an exceptional second half, but the first one we forgot to be there. We are a team that wanted to defend aggressive without the ball, with the ball, and the second half was much much better but it wasn't enough.
"The opening was there, and we will learn. We have a team with not a lot of experience in this kind of competition and it doesn't matter when we play home or away, at that level, when you concede," he added.
"But the second half we had the chances, we had the chances to score a lot, a lot of goals, and we didn't, and that's why we're out."
City went into the game 5-3 up from the first leg but fell to two quick strikes from the hosts in the opening 29 minutes.
Leroy Sane pulled one back in the second half but Tiemoue Bakayoko headed home a free kick to send the Ligue 1 side through on away goals after a 6-6 verdict on aggregate.
Asked if porous defending was the reason behind the loss, Guardiola reiterated that the first session poor show resulted in their decline.
"That's not the reason why," he said. "Of course, we are out for that goal. But it's in the first half, in the first 45 minutes, we were like how we are. That is my only sad thing.
"You can play and the opponent can beat you, but we were not there at the right moment when we should be there."
Guardiola though backed his side to improve.
"I think we'll improve," he added. "We're going to try, of course, to try to improve but this competition is so demanding for the big teams.
"Sometimes you have to be lucky in that moment, but we were not so we will prepare now for the Premier League and the (FA) Cup and think about next season."