Costa was signed from Atletico Madrid for around £32m, while former Arsenal midfielder Fabregas was bought from Barcelona for £30m, and the Portuguese coach thinks that the pair have given his side a new dimension.
Speaking to England coach Gary Neville in an interview for The Telegraph, Mourinho said: "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."
Costa is the Premier League's joint leading scorer, alongside Manchester City's Sergio Aguero. with nine goals this term.