He singled out Espanyol playmaker Joan Verdu as one of the key players for Javier Aguirre's team, and indicated his side would work to limit the damage he can cause in the Primera Division encounter.
"If we want to beat them by playing better than them, it's not going to be easy," he said. "We have to show greater aggression and intensity than them.
"We have to make sure they don't get comfortable, especially their key players, like Verdu. If you allow him time on the ball to think, he can play in (Christian) Stuani or Sergio Garcia and make your life complicated."
Mendilibar has a good record against Espanyol, both with former side Real Valladolid and with Osasuna, and the Navarrans beat Espanyol 3-0 away from home earlier this season when Mauricio Pochettino was in charge of the Catalans.
However, the Osasuna boss does not think these statistics will give his side any advantage tomorrow.
He said: "It's true that I've done well against them (Espanyol) with Osasuna and Valladolid, but this run can come to an end at any time. There are some teams that you fare better against than others, but this is not the same Espanyol that we met in the first half of the season."
A former Osasuna and player, Aguirre told a press conference that the Navarran club will always hold a special place in his heart, but was in no doubt as to where his loyalties would lie once the game began.
"In every club I have been I have left some part of me," he said. "I am very fond of the people I worked with there, and the human aspect always transcends the sporting aspect, but right now I belong to Espanyol and I will fight until the death for my team for the 90 minutes."
The Mexican believes his side will be in for a tough game, especially because of the strong support Osasuna can count on at the Estadio Reyno de Navarra. He also thinks his team will create few opportunities to score, and stressed that they must make any chance they get count.
"It will be a tough and intense game, due to the size of the stadium and the pressure their fans put on you. If you go out onto that pitch relaxed you will create problems for yourself. Teams don't make many chances there and the secret is to not miss the one chance you get."
Victor Sanchez will be missing for Espanyol due to suspension but Joan Capdevila is set to return after two weeks on the sidelines.