The 25-year-old twice played in friendlies for the country of his birth Brazil last year, but decided to represent La Roja after gaining citizenship towards the end of 2013.
Scolari has said that Costa would have been part of his 23-man squad for the tournament, but his place in the Spain party is currently doubtful due to a hamstring injury.
"I was going to summon him for the World Cup," AS reports Scolari as telling the media in Brazil.
"Was I wrong? I don't know. He would be at the World Cup with Brazil. I talked to him twice about it. The problem is there are other interests behind it. With Diego Costa becoming a naturalised Spaniard another space [in Atletico's squad] opened up in Europe.
"There's a whole set of interests. Diego Costa would be with us. He plays as a seven, a nine, or even as an 11. People to try to run him over and go crashing. He's strong as a bull."
Costa scored 36 times for Atletico during the 2013-14 season.