O'Neil says their defending wasn't good enough on the day, leaving him upset with his players on the day.
He stated, “We were too open. Miles too open. We lost structure, lost shape when we had the ball, and we’re just too open, and we weren’t able to defend transition. There were some crazy, crazy moments for their goals and it was a real poor display.
“It was disastrous. A terrible performance. We didn’t play anything like us, not in shape, so open, so a lot to consider. Off the back of the other performances where I thought we were close, this shows me that we’re not where I thought we were and things need to change.
“They’re terrible goals, with bad decisions, bad positioning and so much wrong with them. Players of course make mistakes, but I need to find a way to put them in positions where they make more good decisions and less mistakes.
“We get into the game because we're open and we lose control of the shape of the team. Then when you think we're going to get in at half time, we're going to be able to get hold of them. We then concede a fourth right on the stroke of half time, and it makes the game really difficult from there.
“But it was a really poor display and something that I now have a couple of weeks to digest and have a look at what we can do with this group to make sure that we're more competitive than we were today.”
On the game not going to plan, the manager also said: “I've never seen us that far away from the game plan before. Whether the early goal – which we discussed – rocked us, but whatever it is, it’s not acceptable for that to be the level of performance.
“I've spoken in previous weeks around the level that we managed to reach against top sides in the league, but that today gives you no chance of winning a game if you go out and you give Brentford five goals – you have absolutely no chance. It was really poor, and a lot of work to be done together.
“Myself, the group, we've been trying to put things together in a way that we thought would help the group perform, but today is probably first evidence that it needs a rethink.”