The 25-year-old, who has scored 10 goals in eight Primera Division matches for Los Rojiblancos this season, was granted Spanish nationality in July and the Royal Spanish Football Federation (RFEF) last month asked FIFA for permission to register the player.
He has, however, represented Brazil in two friendly matches this year - against Italy and Russia in March - and FIFA has now written back to the RFEF demanding "complementary requisites" before approving his registration.
The game's world governing body has also asked the Brazilian Football Confederation to provide a certificate confirming Costa has not played in an "official match" for his country of birth.
Despite the wheels seemingly being in motion to facilitate the change, though, Scolari has suggested the player could still have a future with the Selecao.
Speaking ahead of Saturday's friendly international against South Korea at the Seoul World Cup Stadium, the 64-year-old said: "I am the boss and I am going to give the last list of members for the competition in May. All the Brazilian players have chances, and I will call up the ones who are in the best shape."
The national head coach denied reports from Spain that he will try to convince Costa to stick with Brazil.
"It was said that I will fly to Madrid to talk to him," Scolari said. "Spanish papers know nothing - they are used to making up a lot of things."
Scolari, meanwhile, remarked that long-haul trips for friendly games in Asia was not helping his preparations for next year's World Cup on home soil.
"We know these matches are important for a lot of people, but these long journeys make us change our pace, our way of playing," said Scolari, whose side also take on Zambia in China on Tuesday.
"During training, some players were lost and off the pace due to the different time zones. We are conceding a big advantage."
Scolari confirmed his front line against South Korea would comprise Zenit St Petersburg forward Hulk and Neymar, the Barcelona star passed fit despite having struggled with a hip injury in the build-up.
"At first, I will field most of the starting XI that won the Confederation Cup. I have to respect that," he said. "Then, I will make some changes if they are needed. I have a lot of things to see ahead of the World Cup.
"Hulk gives me more power in the attack than Ramires. Evaluating the opposition, I think Hulk could make a difference."