Coach Vicente del Bosque took a gamble and included the Atletico Madrid ace in his final squad despite the 25-year-old forward struggling to recover from a muscle injury he sustained during last month's Champions League final defeat to city rival Real.
Del Bosque is hoping that Costa – who scored 36 goals in all competitions last season for his club side – will spearhead the team’s attack ahead of Fernando Torres and David Villa.
Although his recovery has been slow, the Brazil-born attacker trained normally with the Spain squad on Monday ahead of its final friendly before the World Cup and struck an impressive left-footed goal during the team's latest practice session.
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When asked whether La Roja would be handicapped if Costa was not fully fit for the tournament, Del Bosque told TOI: “People asked the same during the  European Championship. We were the champions despite playing, as they said, 4-6-0. Costa has played really well this season. He is the man in form and we shall love him to be on the field for Spain.
“But if he is not fully fit, I'll have to take a call before I announce the final team. However, I sincerely believe he will be fit before the World Cup.”
Spain will be looking to retain the title it won in 2010 this summer in Brazil, although the team will face stiff competition from the home side, which defeated the Spaniards in the Confederations Cup last year.
However, Del Bosque has called on his players to first concentrate on getting out of the group – which includes Chile, Australia and Netherlands – before then turning its attention to lifting the World Cup again.
“We need to qualify first for the knockout round," the former Real Madrid boss said. "That is the first target. Once we achieve it, we shall be able to think about our opponent. One thing is for sure: you cannot pick and choose your opponents in a World Cup. And you need to play and win against bigger opponents in the knockout stages to become world champions.”
He added: “I believe we can [win the World Cup again]. They [the team] are older with experience. To win the biggest tournament, you need a proper balance of youth and experience – and we have it.
"The players may have won three big tournaments on the trot, yet, they are hungry to win more."