The Catalan giants head into the clash against their arch-rivals on the back of successive defeats.
A 1-0 loss at Atletico Madrid knocked them out of the UEFA Champions League quarter-finals and they lost by the same scoreline at Granada on Saturday.
That result leaves Barca four points behind La Liga leaders Atletico Madrid with just five matches to play, making their chances of defending their crown unlikely.
One trophy they can win is the Copa del Rey, though, and despite their recent hiccups, Iniesta says his side will take encouragement from their 4-3 league win at the Bernabeu last month.
"We are convinced that we can (win). That's the way it should be," Iniesta said on Tuesday.
"This is the Copa del Rey final and obviously we have to play the best we can against a very powerful opponent.
"The team has every ability to play well and win tomorrow. And we are going to go out and do it.
"The team and the players, we've done things very, very well this season at times against great teams.
"This is the chance to win a title and bring our fans happiness. There's nothing we should be thinking about apart from winning this title."
Iniesta acknowledged recent results were disappointing but said now was not the time to reflect on their failings.
"In the Champions League we have been knocked out and in the league we took a step in the wrong direction on the weekend," he added.
"It's not the time to analyse the season (though).
"We can do that at the end of the season and decide if it has been a good season or a bad season.
"The important thing here is Barca are always competing for titles at this time of the year.
"Tomorrow it's up for us to play in a cup final and go back to the league and keep fighting until the end."
