Athletic need a win for any chance of qualifying for the Europa Conference League.
"It's possible that we have our chances and we have to fight until the end. That's what we're convinced of," said Valverde.
"In football you write a lot beforehand, but you never know what's going to happen. That's the good thing about it.
"It's true that it's more difficult for us than it was a few games ago, but we have to play.
"We know it's difficult, because of the opponent we have in front of us, because there are other games and because we don't depend on ourselves.
"We must focus our energies on what really depends on us. Everything is reversible and of course it's a definitive match."