Barcelona skipper Andres Iniesta insists they can still win the Spanish Supercopa despite their humiliating 4-0 first-leg defeat.
The Catalan club were stung by a second half Aritz Aduriz hat-trick and a jaw-dropping 50-yard strike from defender Mikel at San Mames on Friday.
Luis Enrique's side now have a mountain to climb on Monday at the Nou Camp if they are to claim their sixth trophy of 2015.
Speaking to Barcelona's official club website, Iniesta insists the tie is not completely over for Barcelona.
"From the first minute we leave thinking we can turn around the tie," he said.
"It's complicated but it is not impossible. We must believe we can do it. We made many mistakes and we have been penalized with a heavy loss.
"When things do not go your way you have to improve and not let it happen again."
The crushing defeat came three days after Manchester United transfer target Pedro came off the bench to earn the Catalans a 5-4 victory in a tumultuous UEFA Super Cup final in Tbilisi.
But Enrique refused to use the exhausting contest, which was settled in extra time, as an excuse for his side's slump against Bilbao.
"This is no time for excuses, we must try to improve our performance," Enrique said. "The result we cannot change. On Monday the atmosphere will be in our favor and we try to give back to our fans.
"If anyone can come back in this tie, then it's Barca."