Jack Grealish struck for Everton midway through the second-half for the only goal of the evening.
Iraola said: “I think it's a very flat performance. Poor performance, as you want to call it.
“I think we played more the game that Everton wanted to play than the one we wanted to play in the sense that not a lot of things have happened and we are less comfortable in those kind of games.
“And I was worried a little bit because it was not happening a lot of things but set pieces and these things you always can concede has happened in a transition.
“But I think we've been, especially on the ball, I think we've been very poor today. I think our build up has been, we haven't progressed, we haven't had, I don't know, personality or the quality to progress easier on the ball.
“And then when you are not good on the ball, your press, everything looks worse in the sense that you lack energy and everything looks more difficult.”
Asked why they managed just five shots on goal for the night, Iraola said: “Because of the rhythm of the game.
“I think in a game where everything was going almost like slow motion. I think we are not the best in this kind of games.
“We like, or we are better, in other scenarios and we haven't been good enough to get the scenario that we wanted especially for me because we've been not taking the risks on the ball or playing too safe or not progressing enough on the ball.