The striker was left out of Spain's latest international squad and spent time with the Blues' physio instead, which has seen him freed of his physical troubles
Costa was omitted from Spain's squad for their games against Belarus and Germany in the most recent international break due to persistent trouble with his hamstring.
Mourinho had criticised Vicente del Bosque's decision to play the former Atletico Madrid striker in Euro 2016 qualifiers earlier in the season, saying the 26-year-old needed to be given time to recover.
And the Portuguese told his media conference on Friday that Costa has recuperated fully over the last fortnight, and is raring to go for Chelsea's Premier League clash with West Brom on Saturday.
"In these two weeks he has had a little bit of a holiday, but one spent with a Chelsea physio," said Mourinho.
"Even in that small period of holiday he was working many, many hours per day with our physio. After that he came back, he keeps going in the right direction and, right now, we feel he's free of his old injury."
Mourinho will continue to look after Costa in the coming months, though, with the striker preparing for his first taste of a season without a mid-campaign break - as is commonplace around the rest of Europe.
"I believe [this period has set him up for the rest of the season], but now we are going to have a period that is new for him," Mourinho added.
"I always think that the Christmas period for players doing it for the first time in their careers is more difficult.
"The English players, or those who have been here for many, many years, are more physically and, especially, more mentally adapted and prepared to cope with that incredible marathon of football we have to face.
"With Diego, we have to go match by match, but the way he trains and expresses himself in training, he has a free mind. He's free of fears, at this moment. He is good."