Portugal have used the same starting line-up in all four of their games so far but Postiga has been ruled out for the rest of the tournament after suffering a hamstring injury in the quarter-final win over the Czech Republic.
Almeida, who plays for Besiktas and featured as a substitute when Postiga was taken off, said he was ready for the challenge.
"Yes I am very pleased with that [call-up] I have worked a lot for that and I will take this opportunity," he told a news conference on Tuesday.
"While I was out of the side, I stayed motivated, like all the 23 players in the squad," he said.
"All of us want to play all the time. Of course, I was a little sad not to be in starting line-up but I did all I could to help my team - you have to work hard and be prepared to take your chance," he said.
Spain's defence has been watertight throughout the finals, conceding just a solitary goal in the opening game against Italy and Almeida, who has 15 goals in 43 games for his country, has no doubt about the difficulty of the task facing his team.
"We have to score, that's the first aim, we know they have great quality, stunning players, and we will have to continue our work and try everything to score," he said.
With Cristiano Ronaldo and Nani on the wings and Almeida playing through the middle, Portugal's approach is unlikely to change from the counter-attacking style they have deployed so far, despite the unique challenges the Spaniards pose.
"They have an identity based around the ball possession they have," said Bento. "They always play in last third and are very strong when they lose the ball, that's why they are a team of great quality, why they are European and world champions.
"We will have a lot of difficulties but the important thing is that we have to be loyal to our identity too," he said.
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