Atletico Madrid striker Costa, who is the leading scorer in the Primera Division with 10 goals, is eligible to represent either Brazil or Spain but it is not yet known which nation he will opt to play for.
Del Bosque concedes it is not an easy situation for the 25-year-old, who has played twice for Brazil in friendlies, but believes a final decision will be made soon.
"We are going to have a conversation with him. He's expressed his desire to us and he's in a difficult situation because Brazil also want him and he has to decide. In the next few weeks he will decide," Del Bosque said on TVE.
The Spain boss also played down suggestions that calling up Costa would represent a slight to the forwards already available to the world champions.
Alvaro Negredo and Michu - the latter drafted in to replace the injured David Villa - were the two frontline strikers in the latest Spain squad, while the likes of Fernando Torres, Roberto Soldado and Fernando Llorente will all be hoping to win recalls ahead of next year's World Cup finals.
Del Bosque said: "Some people talk about other players and say that by picking Diego Costa we're belittling them.
"Every time we name a squad we are belittling someone, because there are a lot of players who could be in there.
"You cannot make a list that satisfies everyone."
Spain's 2-0 win over Georgia on Tuesday ensured the holders would be on the way to the World Cup.
The two-time reigning European champions will be among the favourites to lift the trophy in Rio de Janeiro on July 13, but Del Bosque is keen to temper expectations.
"It's difficult to defend the world title in another continent," he added. "It gets harder all the time."