Aguirre has two draws in his pocket already. His defensive line wasn't great, so he decided to build a good defensive team first instead of trying to score many goals. Spanish midfielder Joan Verdú is arguably Espanyol's best player, but Sergio García's creativity could also be dangerous.
Pochettino's favorite striker was Longo, but Aguirre seems to like Stuani better. Both of them are quite good with aerial balls, but neither one has the quality to score frequently. Former Villarreal winger Wakaso runs a lot, but he also commits a lot of mistakes for his team. He could also play in Espanyol's midfield, but that's not his natural position.
Other than that and Mexican defender Héctor Moreno, Espanyol doesn't have tons of weapons. Christian Álvarez is a very decent goalkeeper, but Colotto, López and Álvarez aren't enough for his team to prevent the opposition from scoring many goals.
Javier Aguirre is known for his good defensive strategies, though, so Real Madrid could have troubles trying to break Espanyol's defensive line given how bad the team is performing with the possession of the ball. As always, it will be a midfield battle. If Espanyol's Forlín and Sánchez are capable of stopping Real Madrid's playmakers, it will all come down to Real Madrid's accuracy.
If former Atlético Madrid winger Simao Sabrosa plays, Espanyol would have a very dangerous threat in their set pieces. And Real Madrid's poor defending in these plays is something Aguirre's team should take benefit from.