Arnold was referring to the drop in form and consequent lack of results in the middle nine rounds of 2015-2016 that changed the Sky Blues' season and eventually would rob them of a place in the finals.
Arnold's concerns may be materialising after a third straight performance that had lots of grit and determination but lacked the finesse, sophistication and ruthlessness that have become the hallmarks of a side that are still unbeaten and leading the competition.
In their two previous matches Sydney had to fall back on their defensive steel to beat vibrant Brisbane Roar 2-0 and they were given an almighty fright by battling Central Coast Mariners who were unlucky to lose 3-2.
Sydney were nowhere near their best in the goalless derby against Western Sydney Wanderers which was as passionate and uncompromising on the field as it was in the stands.
It was yet another fierce and controversial contest as we have come to expect whenever these two bitter rivals clash.
It's just a shame that the quality of football on show at a packed Allianz Stadium was rather poor and unadventurous.
It was a typical derby with no quarter asked or given but both teams at times appeared to be more intent on not losing than winning and they ended up cancelling each other out.
With second-placed Melbourne Victory enjoying a rich vein of form, Sydney will have to lift their game if they are to emerge unscathed from the much expected clash with Kevin Muscat's side at Etihad Stadium on January 26.
Sydney are five points clear of Victory but the latter have a game in hand.
Sydney had a lot of ball during the derby and on a few occasions they looked like breaching the Wanderers' defence but they never really got going all night against a side that were an improvement from the one that crashed 4-0 in the season's first derby at ANZ Stadium
Playmakers Milos Ninkovic and Milos Dimitrijevic had an off day - their final ball was often disappointing - and were overshadowed in midfield by the Wanderers' star import Nicolas Martinez who had a blinder.
Unfortunately for the Wanderers, coach Tony Popovic has such a sterile attack that Martinez's inspired promptings and positive approach often came to nothing.
Popovic must find a quality striker as soon as possible if the A-League's draw specialists are to nail down a finals spot and make a mark in the forthcoming AFC Champions League.
Arnold, who led Central Coast Mariners to the championship in 2013, bluntly refuted a suggestion in the after-match press conference that he must be happy with a draw.
"I'm happy with the performance but not with the result," he said. "We wanted to win."
Coaches tend to give their own spin when it comes to assessing matches but I would have thought that the other way round might be a more accurate appraisal.
Sydney did not play too well in the middle and front thirds and, with respect, holding out the toothless Wanderers is no significant achievement.
The Sky Blues effectively created only one meaningful scoring chance when Alex Wilkinson's header was acrobatically tipped over the bar by Vedran Janjetovic in the first half.
A better team than the Wanderers could have punished them so it would appear that Arnold should be less content with the performance and more than satisfied with the result, particularly since the Wanderers were denied a clear penalty when Michael Zullo handled the ball in the box in the first half.
Sydney no doubt are a top side that can play much better and more efficiently than they did in the city's 14th derby and they have a chance to get back into their winning stride when they take on Adelaide United at home on Friday.
Victory over the champions in a style that their fans have grown accustomed to this season would suggest that their current slump is a mere aberration, a minor glitch that should not stop them in their tracks.
Yet another unsatisfactory performance, regardless of the result, would raise a few eyebrows and confirm that Arnold's men have gone off the boil and his early-season fears were fully justified.
It also would make free-scoring Victory favourites to win the blockbuster in Melbourne that could have a strong bearing on the outcome of the race for the Premiers' Plate.
If results go their way Victory could be the new league leaders by the time the Australia Day clash is over.