The midfielder believes the referee was the reason behind the 2-0 Champions League setback to Barcelona on Wednesday evening
Two goals from the brilliant Lionel Messi give the Catalan club a big advantage heading into the second-leg with the game full of controversy as three red cards were shown.
Barcelona's substitute goalkeeper Jose Manuel Pinto was handed a red card at half-time during a tussle between both sides before Pepe's lunge on Daniel Alves saw the Portuguese player dismissed.
Jose Mourinho was sent to the stands for his reaction to Pepe's red card and Xabi Alonso agreed with his coach's reaction.
"Until the sending off we had not seen anything new from Barcelona. Referees need to make these decisions more carefully because it's a Champions League semi-final and the score at the time was tied," the midfielder told reporters after the game.
"His decision [to send off Pepe] was completely decisive. We had chances but the game was as he wanted."
The Spanish World Cup winner stated his team-mates are very frustrated with the result as progress to the Champions League final looks to be off the menu for Mourinho's men.
"We're very frustrated because the referee's decisions took the game beyond out control. We will go there [Camp Nou] and fight for it but it will be very, very difficult," he added.
The two sides meet again in Barcelona next Tuesday where Madrid will be hoping to pull off a great comeback to reach the final for the first time since 2002.