Fortress Allianz crumbled as Brisbane Roar ended Sydney's remarkable unbeaten home run in the league.

The 2-1 victory by Roar means the harbour-siders have won just once in seven matches - domestically and in the Asian Champions League - and sit just five points above Newcastle Jets with four games remaining.

All three goals came off set pieces.

Arnold said it was "disappointing to lose" the game but dismissed talk of a form slump as the Sky Blues attempt to secure back-to-back premierships.

"Again we conceded two set pieces (but) we haven’t conceded in general play for a long time," he told Fox Sports. "We’ll go away, we’ll look at the video, we’ll learn from that and bounce back next week...

"We obviously practice defensive set pieces, attacking set pieces every week and at the end of the day it’s just about people doing their jobs.

"We’re a team – it’s the staff as well as the players – and we’ve got the character in this squad to bounce back from this."

Matt McKay and Jacob Pepper had Brisbane two goals up within the first 20 minutes before Milos Ninkovic dragged one back for the home side on the hour. A rare goal by Sydney's Alex Wilkinson in the first half was ruled out for offside while Matt Simon hit the woodwork in the 88th minute.

Arnold added: "I think our form has been quite good.  It’s obviously a challenge at the moment with the two competitions but it’s a great challenge for everyone – staff, players, everyone included.

"A lot of lessons are being learnt and it puts players in a good position afterward to reflect on those type of experiences.

"We’ve got that buffer at the moment and we’ll be back in training tomorrow and we’ll work from there."

With the team sitting last in their Asian Champions League group, and results not coming domestically, team cohesion is under scrutiny. And when Sydney's David Carney stormed from the dressing room at half time it didn't go unnoticed.

Arnold played down the incident.

"(Carney) got replaced at half time, he wasn’t happy about it, but he went and put his bag in the car in the carpark and came back in and watched the rest of the game (from the dressing room)."