The Bees won 2-0 at Dean Court through goals from Fabio Carvalho and Igor Thiago.
“I think we haven't been at the level that the competition required,” he said.
“If you want to go far in this competition, you have to give a lot in these cup competitions and we haven't been ourselves.
“We were coming, I think from two very good games and today has been a step backwards. We've changed a lot, the team and probably when it doesn't work, I am the first one to blame.
“But we've lacked some aggression without the ball in our press, with the ball to be more vertical to this sense of urgency. I haven't felt it today.
“I finished very happy the other day how we, especially in possession, moved the ball. But today we've been very far from that level.”
Too many changes
On why they struggled on the night, Iraola said: “I think probably I changed too much.
“I think there are a lot of players know that still they have to grow a lot to improve, to get more ready to the level that we are going to demand from them.
“It's difficult because once the competition is already going and you want to give minutes to the players, but you still have to continue competing.
“But we have to do it like this. There is no other way.
“We've already passed the preseason, so now every trial has consequences. And today we could say that we haven't collectively played well from our side.”