The former Liverpool midfielder believes the referee was the reason behind his side's 2-0 defeat to Barcelona in their Champions League semi-final
A brace from Lionel Messi hands Pep Guardiola's side a comfortable advantage heading into the second-leg but the game turned in the second half following the dismissal of Pepe for a challenge on Dani Alves.
Jose Mourinho was sent to the stands for his gesture towards the officials following the decision and Alonso feels that incident changed the game.
"Until the sending off [Pepe] 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 [Wolfgang Stark] 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.