Although he has been in prolific goalscoring form for the Blues since his summer move from Atletico Madrid, Costa has been struggling with a hamstring problem since last season and Chelsea boss Jose Mourinho revealed recently the 25-year-old was barely training.
Mourinho would also have preferred to see his player rest during the international break in order to receive treatment.
Costa, nevertheless, was called up by Spain coach Vicente del Bosque for the upcoming qualifying double-header, and the Brazil-born forward insists he is ready to contribute.
"I'm here ready to play," h e said to Spanish sports newspaper Marca after linking up with his international colleagues on Monday
"I've had some problems but it's getting better.
"I've already played three games in a week and the coach (Del Bosque) knows that I carry some problems but I will treat them during the week."
However, Costa also says he will not take any risks with his fitness
He added: "I don't want to be in the middle (of Spain and Chelsea)
Mourinho has not told me anything
"One has to take into account that many times I am the one that makes the decision depending on how I feel.
"Here I have the confidence of the coach, the technical team, and if there is a problem I tell them.
"If I'm here and I'm injured then I don't play because it would be worse."
Costa, who made his Spain debut in March in a 1-0 friendly win over Italy, has yet to score for his adopted country after five matches but he is hoping to finally make an impact with the two-time defending European champions
He said: "What I want more than anything is to succeed here (national team)
Up to now things haven't gone as I had thought they would or as people expected.
"This is why I really want to do a good job with Spain.
"I haven't scored but if you look at the games I've played, I haven't really had many opportunities to do so.
"I need to work hard in order to score my first goal, something which would make me very happy."
Costa moved to Chelsea from Atletico in a £32million transfer deal this summer and he has not disappointed.
He scored his ninth league goal in Sunday's 2-0 triumph over Arsenal that lifted Chelsea five points clear at the top of the Premier League standings.
"Things are going very well, but not just for me but for Chelsea," Costa said
"Against Arsenal it was complicated but we managed to win."
Source : PA