The Celta Vigo coach will head to the Santiago Bernabeu with his squad on Saturday evening knowing the enormity of the task.

Even though Jose Mourinho's side have been hit with a lengthy injury list, Harrera does not think it will affect them.

"This is the best Real Madrid in history," Herrera said.

"Last year they were the top scorers, they beat you by knockout. This year they have started differently but they are recovering their style."

The 58-year-old then reeled off a list of potential hazards to his team's target of leaving Madrid with a surprise victory.

"They have the best goalkeeper in the world, Iker Casillas, the best central defender, Sergio Ramos, and one of the best central midfielders, Xabi Alonso," he said.

"Cristiano Ronaldo? That's different, him and Lionel Messi are the two best."