The Atletico Madrid star, who looks to be heading to the Premier League to join Chelsea following this month’s World Cup, suffered a hamstring injury that saw him limp out of both a final-day Liga decider against Barcelona and the Champions League final.
|World Cup Infographics|
|Uncover the statistics behind the stars heading to Brazil with our new Unibet infographic series|
"I needed the game the other day to get over the feeling that my injury would come back,” he told Marca.
"I felt good in those two games [for Atletico Madrid], but I played and it came back… I feel much better – also in terms of my mood and enthusiasm, but the main thing is the injury.
"I'm fully fit and there won't be any problems."
The Brazil-born striker’s controversial decision to snub the nation of his birth in order to represent Spain at the World Cup has made him the villain of the tournament for local supporters, but Costa has been humbled by the support he has received from his new team-mates.
“I have the support of lots of people who understand what happened," he said.
“My team-mates treat me in a very special way. I feel very comfortable. It has been amazing. I'm really experiencing a beautiful moment. I have to be grateful for that."
Spain open their World Cup campaign against Netherlands in Salvador on Friday.