December 10, 2016 will not be a date that endures in the final analysis of Pep Guardiola's illustrious coaching career but it is likely to burn sharply in the Manchester City manager's mind.
Following a chastening 3-1 loss to title-bound Chelsea the previous weekend, Guardiola's men were 2-0 down inside five minutes and 3-0 behind within 20 at the King Power Stadium before Jamie Vardy completed a hat-trick in a resounding 4-2 win for Leicester City.
It proved to be the last hurrah of Claudio Ranieri's magical Leicester stint, with Vardy and Riyad Mahrez fleetingly rediscovering the form that stunned the Premier League in 2015-16.
But they had accommodating opponents. City were truly abject.
During a frantic opening to the match, it was broadly clear Guardiola was fielding a back three but less obvious who was playing in it. Was Pablo Zabaleta supposed to be in holding midfield? Why was Aleksandar Kolarov leaving so much space in behind for Leicester's livewire attack?
A confused, listless display encouraged gleeful criticism of a supposed tactical genius. Guardiola had been brutally undone and looked a man bereft at a post-match news conference where he invited further scorn.
Asked whether statistics showing his team failed to make a single tackle in the opening 35 minutes demonstrated a lack of fight, Guardiola provided a now infamous response.
"I am not a coach for the tackles, I don't train the tackles," he said.
"What I want is to try and play good, score goals and arrive more. So, what's tackles?"
The Manchester City side who face Manchester United at Old Trafford on Sunday, exactly a year on from the capitulation at Leicester, are runaway leaders targeting a Premier League record of 14 consecutive victories in the same season.
They are unrecognisable from the rag-tag assortment who toiled in the Midlands rain and a testament to a man who held on to his nerve and principles when faced with failure and ridicule.
The personnel has changed, with the deadwood shipped out as significant as the new faces brought in at considerable expense.
Zabaleta, Kolarov, Fernando, Bacary Sagna and Jesus Navas are all gone from the starting line-up at Leicester, as is Nolito, who came on to score a late consolation. Claudio Bravo turned in one of those performances that have left him marooned on the bench this season.
Gabriel Jesus arrived from Palmeiras the following month, while Leroy Sane would only hit form at the turn of the year.
City reached the FA Cup semi-finals and achieved Champions League qualification but criticism of Guardiola remained - why was he asking aging players to carry out jobs they were ill-suited to perform?
Sticking to his stylistic guns has been rewarded handsomely this season. If, after Leicester and a similarly humiliating 4-0 loss at Everton the following month, Guardiola had opted for something more prosaic, David Silva, Kevin De Bruyne, Raheem Sterling and Sane would not be combining with the same instinctive and dazzling brio they do now.
Ederson's impressive introduction to English football shows the benefits of a playmaking goalkeeper that the blundering Bravo was supposed to bring last season.
Alongside Ederson, Kyle Walker is likely to be the only pre-season signing in City's starting XI against United. Any perception that Guardiola simply threw money at the ills of 12 months ago and watched the rewards roll in is simplistic and wide of the mark.
His City are a triumph of coaching, of selling a vision to players he has made better. And it's not just the headline grabbers.
Nicolas Otamendi's authoritative displays at centre-back are far removed from the hot-headed liability Guardiola inherited. Necessity might have been the mother of invention when it came to fielding Fabian Delph at left-back but the sight of a fine player revitalised after his career was in a state of drift has been wonderful to witness.
John Stones, so poor that day at Leicester, might have been shielded from a game as important as this Manchester derby back then. But he has become fundamental to Guardiola's team and his absence with a hamstring injury offers United considerable encouragement.
As the City manager and his senior players have been at pains to point out, trophies are not won in December. But beating old adversary Jose Mourinho to establish an 11-point lead at the summit would give them a vice-like grip on the title race.
It is a story of incredible progress and a man living up to his hefty billing as a footballing visionary, even if Opta data shows no side has contested fewer duels than City this season. What's tackles?