The well-respected Sky Blues skipper was booked for dissent during Saturday night's 2-1 upset and then appeared to spit in the direction of referee Kurt Ams.

But a review of the incident has provided insufficient evidence to justify a referral to the full A-League match review panel.

That lack of evidence is likely to centre around an inability to prove Brosque's intent, especially given the 34-year-old was a substantial distance away from Ams at the time.

Sydney declined to comment on the incident, but it's understood the matter is being dealt with internally.

It comes amid badly-timed cracks in the club's previously impregnable stronghold on the competition.

A first A-League loss at Allianz Stadium in 715 days also coincided with a dummy spit from utility David Carney, who reportedly showered, changed and stormed out of the venue after his halftime substitution and had to be chased to his car and convinced to return and watch the second half from the sheds.

The strain of juggling A-League and Asian Champions League campaigns has taken its toll via a run of three losses and a draw from the last four games - a slump the defending champions have not experienced since their trying 2015-16 season.

That slip in form is also synchronised with news that coach Graham Arnold will leave at season's end to take charge of the Socceroos.

All of a sudden, Sydney's claim to a second consecutive Premiers' Plate is no longer a mere formality, with Newcastle just five points adrift and bristling with the confidence of an extraordinary campaign.

It renders this weekend's trip to Central Coast and the following to Perth all the more important.

"Now we have just under three weeks to focus only on the A-League. We have to pick ourselves up now," Arnold said.

"We've had two fantastic years of winning, but everyone goes through a period like this.

"This is a time where everyone needs to step up and I am the first to look in the mirror and say it myself.

"I have to drive these boys forward."

Australian Associated Press