After the sensational comeback against Paris Saint-Germain, Madrid were euphoric. They had defeated one of Europe's elite, humiliated an economic rival, neutralised Lionel Messi and, many believed, convinced Kylian Mbappe.
They were about to win La Liga and suddenly looked contenders in the Champions League too.
Three weeks later, the advantage in the league is still intact thanks to a scruffy, albeit morale-boosting, win over Celta Vigo on Saturday. But Ancelotti's credit has all-but evaporated.
Real Madrid were waiting until Tuesday to decide if their coach will be able to travel to the first leg of their Champions League quarter-final against Chelsea on Wednesday, after he tested positive for Covid last week.
The outcome of the tie could decide if he continues as coach next season.
It says something about the weight of the Clasico fixture that eight months of largely impressive work can be almost entirely erased by one, disastrous, 90 minutes.
There has been criticism of Ancelotti –- of Madrid's counter-attacking style, his lack of rotation and, in particular, the manner of the passive first-leg defeat by PSG –- but a dominant position in La Liga had mostly kept the sceptics in check.
Losing 4-0 to Barcelona, though, at the Santiago Bernabeu, when Madrid could easily have shipped six or seven, has brought doubts to the fore, not least because that result served as a warning for what may be to come.
Barca are 11 points behind Real Madrid in the table but it is Xavi Hernandez's resurgent young side who are finishing the season as the strongest team in Spain.
For Ancelotti, the Clasico defeat means winning La Liga may no longer be enough. How they win the title, with eight games to go, and how they fare against Chelsea will now frame how the first year of Ancelotti's second tenure is viewed.
Real Madrid's president Florentino Perez met Ancelotti over the international break and while Perez reassured Ancelotti his future was secure, the need for talks was significant.
When Ancelotti was sacked by Real Madrid in 2015, a year after winning the Champions League, he later admitted the moment he was told to meet Perez was when he first knew his days were numbered.
Ancelotti's arrival last summer was a surprise, for everyone, including Ancelotti and Perez.
It was only a chance conversation between them that saw the idea floated and then quickly take shape. "It's happened fast," Ancelotti said at his unveiling.
That contributed to the sense that this was a short-term appointment, one that could be curtailed without much fuss or financial penalty.
If there is a parting this summer, it is unlikely to be bitter. With the league title is won, both parties would consider it a success.
But Ancelotti's appointment was also perhaps a reflection of the market last summer, when there was a dearth of top coaches available.
Barcelona looked to replace Ronald Koeman but stuck with the Dutchman after finding nobody else was suitable. Manchester United continued with Ole Gunnar Solskjaer. Juventus turned back to Max Allegri.
This summer, there could be more persuasive options, with Mauricio Pochettino expected to part ways with PSG. Erik Ten Hag may be prepared to leave Ajax and Raul Gonzalez, Real Madrid's legendary former striker, will have completed three years with the B team and could be ready follow the path of Zinedine Zidane.
Depending on the ownership situation at Chelsea, perhaps even Thomas Tuchel could be lured.
Even if Chelsea's future is resolved and Tuchel reassured, his presence in this tie is awkward for Ancelotti, the German representing the type of younger, more progressive coach that Madrid have so far avoided but might need if they are to stay ahead of Xavi's Barcelona.
A positive showing should be enough to convince Perez that Ancelotti is worth a second season, especially with Mbappe expected to join in the summer.
Ancelotti has an enviable track record of getting the best out of top players. His work with Vinicius Junior, who currently plays in Mbappe's position, has been transformative.
But defeat would leave Ancelotti vulnerable, compounding the Clasico loss at a time when his critics are gathering ammunition. He will know there is no margin for error.