Spain head coach Julen Lopetegui admitted it was "painful" to leave Alvaro Morata out of his World Cup squad.
The Chelsea striker was overlooked for the final 23-man group for the tournament in Russia, which was announced on Monday.
Morata scored five goals in as many appearances during the qualifying campaign but his struggle for form and fitness in the Premier League this season left him facing a battle to keep his place for the finals.
Costa, Moreno and Aspas preferred
Lopetegui found the decision difficult but said he did not want to take four players for the centre-forward role, with Diego Costa, Rodrigo Moreno and Iago Aspas the preferred options.
"They are tough moments for a player. It's not nice, he deserved to be here," Lopetegui told a news conference.
"I have the important opinion and it's painful, but I've made the decision.
"I've spoken with those I had to speak to, I'm not going to name anybody. I've had various doubts. To make a list of 23 is a difficult decision.
"We've opted for three other team-mates who have other conditions. Taking four players in that position is not something we wanted to do. It's a footballing decision.
"It's the most bitter taste to leave someone out. They deserve to be there but they couldn't get in. They helped us and they cannot be here. We have chosen those who can help us the most in this amazing adventure.
"Iago can play centrally and out wide, Rodrigo can play in three areas and always has a great attitude with the team. Diego Costa brings a lot of competitiveness."