The world and European champions host Valencia in a semi-final second leg match with Carles Puyol's header from the 1-1 draw at Mestalla last week giving them a slight edge in the tie.
Barca's anxious 2-1 home win over Real Sociedad in La Liga at the weekend ended a run of three consecutive draws, their worst run of results this season, and a rare barren spell for top scorer Messi.
David Villa, Pedro, Andres Iniesta and Alexis Sanchez have all been laid low, forcing coach Pep Guardiola to fall back on their famed youth academy of late, but the zip has been missing from the side's play.
"We'll see if, little by little, we can start getting players back," Guardiola said on Saturday, when he rested other key players such as Xavi, Gerard Pique and Eric Abidal.
"The players we have available are making a titanic effort, and they have a chance on Wednesday to secure a place in the final for later in the season, while we turn our focus on the other two competitions [La Liga and the Champions League].
Barca won the cup in 2009, and lost in last season's final to their arch rivals Real Madrid, who they trail in the league by seven points.
Unai Emery's side, cup winners in 2008, were held 0-0 at Atletico Madrid on Sunday. Aside from cup games they have yet to win in 2012.
Mirandes, who have an annual budget of just over 1.2 million euros compared to the 60 million euros or so at Bilbao's disposal, grabbed a lifeline with Ander Lambarri's injury-time goal.
