Sydney coach Frank Farina stopped short of labelling his side's performance against Melbourne Heart as the worst during his time at the club, but conceded they were flat in the 3-1 loss on Sunday.
The Sky Blues never really got going and were down 2-0 by half-time after goals to Nick Kalmar and Jonatan Germano.
Any hopes of a comeback were dashed 10 minutes after the break when Sebastian Ryall conceded a penalty and was sent off for denying the Heart a clear goal-scoring chance.
Aside from Joel Griffith's cracking strike shortly after conceding the third goal, Sydney failed to create any meaningful chances and Farina knows they have to improve in the remaining five weeks of the season.
"It's not our worst (performance) since I've been here but I thought we were a bit flat," he said.
"We gave them a two-goal lead which is very difficult to come back from especially (after) you get the penalty and the red card to make it three.
"But I thought the second half we were better than what we were in the first half.
"But we as a team have to improve with five games to go."
Sydney have now conceded goals in their past nine matches and Farina said their leaky defence can be attributed to the constant changes that he is forced to make to the back four.
"Goals against have been a concern for us right from the start of the season. Unfortunately we've had to tinker a lot of things with our defensive structure and it's going to happen next week with Seb Ryall," he said.
"Without making excuses, there are reasons why we have conceded a lot of goals and that's just stability in the defence.
"Defence is the cornerstone of your team and if you've got to change that every week it's no surprise that you're going to leak goals."
The acquisition of Socceroos captain Lucas Neill could help bring stability to Farina's back four with the 34-year-old performing solidly in his debut on Sunday.
"OK for his first game. I knew he would and could last 90 minutes, bit of a baptism of fire coming into this game," Farina said
"Relatively pleased and he'll just get better."
Goal-scorer Griffiths was subbed shortly after scoring his goal, but Farina said it was just tightness and he should be okay for next week's trip to Perth.