The Primera Liga leaders have a 2-0 advantage from last month's first leg at the Nou Camp but in contrast to Barcelona's recent poor showing, Mallorca have won their last three league matches and moved out of the relegation places into 16th.
Barca defender Silvinho said the poor results, including Sunday's 4-3 loss at Atletico Madrid when the Catalans twice surrendered the lead, would not affect their performance.
"This is a different competition and we won't be suffering from any damage to our spirits because of what happened on Sunday," the Brazilian said on Barca's website.
"It could be a problem if we think about the result in the first leg so we have to play our normal game and concentrate on attacking to win the match but it will be difficult."
They were confident of reaching May's final, he added, when they will face either Athletic Bilbao or Sevilla, who play in Bilbao earlier on Wednesday.
Barca's advantage over champions Real Madrid in the league was 12 points when they won the Cup first leg on Feb. 5 but the form that helped them amass 65 goals and concede just 15 in the 21 league matches up to that point has deserted them.
They now lead Real by just four points in the standings and Mallorca's Venezuelan forward Juan Arango, who scored the winner in the club's 1-0 win at Sporting Gijon on Sunday, said Barca were clearly not invincible.
"We are just one step away from the final and we will make an incredible effort to get there," he told the club's website.
Sevilla won the first leg of their tie against Athletic Bilbao 2-1 and came back from a goal down to win 2-1 at the San Mames in the league on Saturday.
Sevilla striker Frederic Kanoute scored a stylish double in that match against an understrength Athletic but said Wednesday's game will have a completely different complexion to their league clash.
"But we are confident and they will be more nervous than us," the Mali international said on the club's website. "We have a comfortable advantage and we won't be intimidated by the atmosphere."
