"The quality of football we produced is amazing. We never lost a match because we didn't play or that the opponent was stronger, it's just we don't kill opponents and we give them life," Mourinho told Sky Sports after his team were knocked out in the quarter-finals.
"Every opponent knows they can score a goal (against us). We had fantastic chances to score and didn't score."
While Chelsea sit third in the Premier League and are safely through to the knockout stages in the Champions League, surprise recent results have forced the Mourinho on to the defensive about his team's chances of silverware.
Two defeats and a draw in their last seven Premier League matches as well as a double reverse against Swiss side Basel in Europe have turned the focus on the club's mis-firing strikers, Fernando Torres, Demba Ba and Samuel Eto'o.
The trio have managed five league goals between them all season.
Eto'o was handed the responsibility for leading the line against Sunderland and wasted an excellent chance to double the lead in the second half when he side-footed wide with only the keeper to beat.
Asked why his current Chelsea team is not as ruthless as the one he had in his first spell with the club, Mourinho said: "I can't answer this question. I'm the manager of this team not the manager of the old team. We are not behind the old team, but in front in terms of quality.
"But football is about results and about scoring goals. There were many key moments not just one. We always wanted to go through in this competition but it wasn't a priority. We have to score goals and win matches, not (necessarily) to be best team in every game."
Chelsea have a testing run of fixtures over the festive period, travelling to league leaders Arsenal on Monday before hosting Swansea City and Liverpool.