The A-League champions slumped to a 2-0 loss to Kashima Antlers at home on Wednesday, leaving them with one point from three games.

Kashima had rested a series of key players but still proved too strong in Sydney.

"You're playing Champions League, it's not mickey mouse," Arnold told Fox Sports.

"You're up against the best in Asia, the best teams and the best players.

"The Japanese have got squads of 35 players, they can leave a few at home and they've still got fantrastic players.

"The mobility was great, their strength was great, technically they're very good. We'll learn from that."

Arnold said Sydney lost because they failed in the big moments, with Adrian Mierzejewski spurning their best chance, while they conceded from two set pieces.

"It was a game of big moments," he said.

"I'm disappointed we conceded two soft goals off set pieces.

"Other than the last five minutres when we opened up, they had two chances.

"Before that we had two great chances and it could've been a different game."

Sydney are next in action in the reverse fixture against the Antlers next week but the Sky Blues have headaches with issues for Michael Zullo and Brandon O'Neill.

"We've got to stay posiitve and recover well," Arnold said.

"The most important thing is we stay positive and take a lot from this game."