The west London side are five points clear at the top of the Premier League table after a 2-0 victory over title rivals Arsenal before the international break.
This season we improve footballistically, with Diego and Fabregas, no doubt
The former Atletico Madrid striker has scored nine league goals since his £32m move, whilst the ex-Arsenal man made six assists following his controversial switch.
And Chelsea manager Mourinho believes Costa and Fabregas have helped the Premier League leaders develop into a more stable side ahead of their trip to Crystal Palace.
“We had certain limitations in the team in terms of tactical qualities, technical qualities, and we were aware of that,” Mourinho told Daily Telegraph.
“My style of leadership is not a style. I try to have a leadership that is adapted to the reality. And last year I was feeling that they were not ready for what I call a pressure leadership – or confrontational leadership.
“The team as a team was mentally – and even tactically – unstable.”
Mourinho added: “So my team was unstable. This season we improve footballistically, with Diego and Fabregas, no doubt.
“When we analyse in tactical and technical terms, they represent the kind of player we need, the kind of second midfield player, the quality of striker.
“We were lucky to have in the market available for us exactly the style of player we need. But what people maybe don’t realise is that the maturity of our team, the personality of our team, changed a lot.”
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