The Portuguese, who will play France or Spain in the semi-finals on Wednesday, hit the bar twice in the second half of a closely-fought 1-0 group-stage defeat by the Germans.
An exhilarating 4-2 win over Greece on Friday cemented Joachim Low's side's status as tournament favourites along with defending champions Spain.
"When it comes to the match against Germany, we are a different squad now," Costa told reporters at the team's base in western Poland.
"I think that in the second half we proved that we can beat the Germans and I think the result wasn't fair in the end.
"But let's not speak too much about Germany because we don't know if they'll get to the final and we don't know if we get to the final, so let's see what happens."
Germany will play England or Italy, who play in Kiev on Sunday, in the second semi-final on Thursday.
The Portuguese have stressed they are not concerned which team emerges from the third quarter-final in Donetsk on Saturday.
Apart from a 4-0 demolition of a weak Ireland side, however, world and European champions Spain have struggled to find an attacking edge in the absence of injured striker David Villa.
"[Spain] are a very powerful team, they are based on Barcelona in their style of play, with a lot of passes and ball possession," Costa said.
"It is not very direct football and currently they are playing without a striker and some say they are struggling a bit with their tactics, I hope they keep on struggling."
"As for France, they are a very young squad, but a very talented one, direct football, fast football. They also have a lot of creativity, so they are a very difficult opponent for us if we are going to meet them."
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