The odds are firmly stacked against Sergio Gonzalez's men ahead of their short journey across Barcelona to Camp Nou, with their recent La Liga record against their illustrious neighbours inspiring little confidence.
Barcelona and Espanyol have faced each other 25 times in the league since 2001 and the latter have only come out victorious on two occasions in that period.
Espanyol's current form will not provide much reason for optimism either, with Sergio's side winning just one of their last six in the league and none of their previous 10 away games in Spain's top flight.
Nevertheless, Casilla is feeling positive and has found himself pondering the possibility of claiming a famous victory on Sunday.
He told reporters: "To win at Camp Nou and with it being a derby, it would be one of the most beautiful things that could happen. It would be great.
"All teams have some weaknesses. They have few, but they do have some and it is necessary to take advantage of those to claim a positive result."
Barcelona on the other hand come into the match in good form having won their last six games in all competitions, with the 5-1 crushing of Sevilla and a smash-and-grab 1-0 triumph at high-flying Valencia particularly catching the eye.
The last-gasp victory at Valencia last Sunday kept Barca just two points behind pacesetters Real Madrid and Luis Enrique has been able to keep the majority of his stars fresh for Espanyol's visit, with many of them rested during the 4-0 Copa del Rey win at Huesca.
Luis Enrique may well opt to hold some of them back once again on Sunday, with a must-win UEFA Champions League clash against Paris Saint-Germain on the horizon.
Though, Marc Bartra, who impressed against Huesca, is adamant that whoever starts for Barcelona on Sunday will be fully motivated.
"All the matches are important, but it's true that we are really up for games like these," he told the press.
"They are tests that if we win them will make us stronger. We have to win, above all try to play well, and the team is in good shape going into these two games."
Both coaches have almost full squads to pick from on Sunday, with Thomas Vermaelen (hamstring) the only player definitely missing for Barca.
Meanwhile, midfielder Paco Montanes (thigh) is a doubt for Espanyol.
‘Maybe I’ll be signed by Wrexham, who knows? English football would suit me – if an offer came, I wouldn’t think twice, I’d go immediately’ Euro 2024 cult hero reveals his UK dream
‘Managing Leeds? It was an option that appeared, but it wasn’t the right timing. I decided it wasn’t a good idea to leave the club I was at mid-season’: Premier League boss admits to turning down opportunity to replace Jesse Marsch in 2023
