The Brazilian forward will make his debut for Spain against Italy this week.
"I'm not going to hide the fact that it's a special case. He has never played for us before, he was born in Brazil… he's coming because of what he's achieved with his club. We believe he can help us get better, which is why we've called him up," explained the Spain boss.
Nevertheless, while admitting that Costa is "doing very well" at Atleti, the 'La Roja' supremo stressed that "him being in this squad doesn't mean he'll go to the World Cup. We have to be wary of ripping up the unity of the group".
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