The Blues boss revealed he ran the rule over the 18-year-old Costa when he emerged at Portuguese club Braga in 2006, during his first managerial stint in west London.
Mourinho hailed his £32million summer signing from Atletico as the "humble" striker from "behind the sunset" who can become this term's outstanding Barclays Premier League star.
The Stamford Bridge chief remains adamant missing out on the Premier League title would not represent automatic failure this season.
"To be fair when he was in Portugal I had a look at him, so everybody was blind, including myself," said Mourinho, of the Brazil-born Spain forward.
"He can be (the star of the Premier League), yes I know.
"He's still the guy that comes from a very small village behind the sunset.
"His village is behind the sunset - you want to arrive there, and it's difficult to arrive.
"He's still that guy, the way he behaves, the way he acts, the way he interacts, he's still that naive, pure, simple, humble guy."
Mourinho claimed he will absorb the pressure - both internal and external - for silverware at Chelsea this season, refusing to alter his plans.
"It doesn't change my approach," said Mourinho.
"It doesn't change my vision, my feeling.
"We can do nothing more than say, we can do nothing more than feel
We feel we are title contenders, we can't add anything else to this.
"If the first team is better than us, if we are good, very good, but somebody is better than us, that's football, that's not a failure.
"But the only thing you can do is to be good.
"We have to be good, to be better than last season, in everything.
"We have to fight for titles, and after that if somebody is really better than us, it's because they were fantastic."
New boy Costa can only speak "10 words in English", but Mourinho said his low-key personality and warm body language have already endeared him to his team-mates.
"He's such a communicator, he's such a group guy: he's such a happy man that despite not speaking more than 10 words in English, he still speaks with everybody," said Mourinho.
"He can have a conversation and interact with everybody, and he doesn't understand one single word the person is saying.
"But just because of his gestures and body language, and happiness and so on, everybody likes the guy so much.
"And he comes from Atletico, he comes here, and adapts quickly, he's the happy guy in the group, he's not shy
It's fantastic.
"I say to him every time, 'do you need me to explain something in Portuguese', and he shrugs and says, 'I understand everything'.
"I think football-wise yes (he understands), especially in football meetings and videos, and my comments and so on.
"I think he goes in the direction I want him to go
He's a clever guy."
Source : PA
Source: PA