Willy Caballero said Manchester City need to end their poor run of form after what he admitted was a terrible night for them against Newcastle.
The Capital One Cup holders crashed out of the competition with a 2-0 home defeat to Alan Pardew's side at the Etihad Stadium.
It followed Saturday's 2-1 Premier League loss to West Ham and meant they have now gone three games without a win.
Goalkeeper Caballero was criticised for allowing shots from Rolando Aarons and Moussa Sissoko to beat him.
And he told City's official website: "It is a really bad day for us. For me too of course because we conceded two goals. We have to change this run because it is no good for us."
Caballero is expected to return to the bench for Sunday's Manchester derby, with Joe Hart likely to take over for the pivotal games against Manchester United and then CSKA Moscow in the Champions League.
The Argentine added: "Next week is a big opportunity for us. We have to recover, forget about this match and think now about United."
City emerged victorious in both Manchester derbies last season and Caballero added: "I expect to win this game. We need all the best performance from every player."
Striker Stevan Jovetic admitted City are not proving prolific enough after they failed to score for the second time at home already this season.
He said: "We need to be more determined and we need to score goals, because we are a team that scores more goals than everybody, and when we don't score we have problems. It happened today. I hope the next game we will score again."
Jovetic said City's Capital One Cup exit was not due to a lack of effort after manager Manuel Pellegrini named a strong side.
He added: "We won this last year and we wanted to win it again. Everybody played -- Yaya [Toure], [David] Silva. We wanted to win but unfortunately we didn't make it."