The Brazilian-born Spain international completed his long-awaited move to Stamford Bridge from Atletico Madrid recently, and has been hailed as the man to revive the Blues’ attack.
But even though he netted over 30 goals last term and cost £32m, some have expressed doubts as to whether or not he is truly good enough on the back of his below par summer.
Costa led the line for La Roja in their opening two World Cup games and failed to find the back of the net as the defending champions lost both.
He was then dropped for the third clash in favour of Fernando Torres.
Some of his disappointment was attributed to a hamstring injury suffered at the back end of last season, and Mourinho believes that there is no doubting the fact that he will be a big hit at Chelsea:
“I’ve known Diego since he was 17 and he went to Portugal, he is the kind of guy whose life was not easy, nobody gave him anything for free. He has always had to fight a lot to do things.” He is quoted by The Express.
“In Portugal he was a Braga player and it’s a good team. He then had to go on loan.
“Then Atletico Madrid bought him, but after that he goes to Celta Vigo because Atletico had (Sergio) Aguero.
“Then he had to go to Real Vallecano because Atletico had (Radamel) Falcao.
“He comes back to Atletico when Falcao goes.
“The guy had to fight a lot to arrive there, I know his psychological profile and I think he is the kind of guy who is afraid of nothing and ready for everything.
“The Premier League is difficult and he knows that, he is ready. I don’t think it will be difficult for him to succeed.”