The Spain international signed a five-year deal with the Blues after completing a £32m switch from Atletico Madrid to Stamford Bridge.
That’s why, lots of the players, they are so happy and so surprised, because his quality is even higher than what they expect
Costa impressed as the 27-year-old played a part in all three of Chelsea’s goals in a 3-1 victory over Vitesse Arnhem on Wednesday.
And Mourinho, who led the Blues to a third-place finish last season, talked up Costa’s qualities ahead of the new season.
Mourinho told Sky Sports News: “He’s a great addition for us. He is a fantastic player.
“Technically he is very, very good. That’s why, lots of the players, they are so happy and so surprised, because his quality is even higher than what they expect.
“He is so strong physically and he gives so much effort during matches.”
Costa scored 27 goals in 32 La Liga appearances to help fire Atletico to the Spanish top-flight crown last season.
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