Auckland FC coach Steve Corica has accused his players of complacency, as they lick their wounds after their first defeat in their maiden A-League season.

The expansion team have seen their seven-game unbeaten start abruptly ended 4-0 by visiting Western United at Go Media Stadium, conceding three first-half goals in an inept display that stunned their fans just days out from Christmas.

After opening their campaign with six straight wins, the cracks first appeared last week, when they had to come from behind twice in a hard-earned draw against Melbourne City, but nothing could have prepared them for the onslaught from United.

"It was terrible, let's face it," admitted Corica. "The boys didn't turn up for the first half and you can't allow a good team like that moments in games.

"We didn't compete, we didn't win the second ball and that's what got us in trouble in the first place — it's a long way back from 3-0 down.

"Maybe a bit of complacency settled in and they don't think they have to do the basic work that we expect from them. If you do that against a good team, they will punish you.

"Last week, I don't think we were great in the first half either, but at 1-0, there are opportunities to come back into the game. At 3-0, you don't get opportunities.

"We were just off. It wasn't one or two players off, it was the whole team."

The dream start to their tenure in the Australia football showcase has propelled the Black Knights straight to the top of the competition table, but the margin that stood at four points two weeks ago is now just one. After five consecutive clean sheets, they conceded their first goal in the dying moments of a home win over Wellington Phoenix and six in the two subsequent matches.

"We've had a good run," said Corica. "At some stage... we didn't want it to be this week, especially at home as well.

"The fans turned up to support us and were amazing throughout the whole game, but we disappointed them in the first half and that's disappointing to me personally.

"When you're on a good run, good momentum, you want to make the most of it. Little things like that can turn sour and there is a sour taste in my mouth today."

United's third goal was hotly disputed by the Mt Smart fans, after referee Casey Reibelt seemed to impede Auckland midfielder Jake Brimmer and the visitors pounced on the loose ball to set up Hiroki Ibusuki's strike.

"Normally, when the ball hits the ref, they would stop it," explained Corica. "If the player runs into the ref, who's actually going to make a challenge, you would think the game would stop, but I was wrong — I don't know the rules.

"That was the third goal, the damage had been done before then."

After their smooth transition into the league, the Knights have begun to experience some setbacks, including the loss of key defender Dan Hall to a broken ankle likely to sideline him for 2-3 months. His absence has seen captain Hiroki Sakai shifted into a central defender position, but the reshuffled formation often looked disorganised against United.

"Any kind of change like that will take away from the team," admitted Corica. "We've got depth in the squad and we showed that last week.

"That's something we need to look at next week, whether we continue that way or we make changes, I'm sure there will be a few changes."

Adversity, especially so early in the season, is not the end of the world and often the best lessons can be learnt picking through the debris of defeat.

"We have to learn our lessons," said Corica, who marked one year since his appointment as Auckland FC coach.

"We're a new club, we're up top and a lot of people probably didn't expect that. It's about staying there and doing the right things, and making sure we have a real crack in games — I don't think we did that.

"This is how football is. You have your ups and downs, you have your wins.

"Today was very disappointing, especially to lose by four. We've had a couple of changes — Dan Hall being out, Hiroki moving to centreback — we can use these as an excuses, but I don't think that's what we'll be doing."

The schedule has gifted Auckland FC an ideal opportunity to make amends, as they take on struggling champions Central Coast Mariners at Gosford next Saturday.