Manchester City boss Manuel Pellegrini says defeat at Chelsea is all down to an improper approach to the second half of the game.
The Citizens started the game with five teenage full debutants but still managed to restore parity before the end of the first 45 minutes.
But they were outclassed by the hosts in the second half as they crash out of the FA Cup.
"I feel we had a very good 45 minutes but after the five minutes after half time we threw away the game," Pellegrini is quoted as saying by BBC Sport.
"There was a counter-attack and set-pieces. Chelsea played well and scored the goals.
"We were forced to select that team, we couldn't play another day and didn't have any more fit players. We knew it was a difficult chance to carry on in the FA Cup. We cannot do anything more.
"If we play on Saturday, it is better for us, then we play in the Champions League for England.
"We have six players injured and cannot take any more risks. We played against a very good team and that's why they won."