These are the moments when André Villas-Boas, for all his outward confidence, could be forgiven for wondering how much longer a man with Roman Abramovich's lack of tolerance is prepared to stand by him when the evidence of Chelsea's deterioration is so overwhelming.
His team were not as dishevelled as Arsenal had been against Milan at the Stadio Giuseppe Meazza last week but at times they were not far off. They began badly, took the lead with a lucky goal, then demonstrated the poor defending and decision-making that have so undermined them in the Villas-Boas era. It was desperate stuff and on the balance of play they can count themselves fortunate the damage inflicted by Ezequiel Lavezzi and Edinson Cavani was not even worse.
A 2-0 win on 14 March would be enough for Chelsea to qualify but Napoli's brilliant forwards, aided and abetted by Marek Hamsik, demonstrated a penetrative edge that should encourage them to think they can score at Stamford Bridge. Once again Chelsea's defence were all over the place.
They played with a staggering lack of organisation and, unless there is a dramatic improvement, they will almost certainly be eliminated. If that happens, English football will not have a side in the quarter-finals for the first time since 1996. Only three teams have overcome a two-goal first-leg deficit in the Champions League.
Villas-Boas insisted he had not picked the wrong team and talked of his confidence they would be better next time. "We have to focus on re-organising the defence," he said. Yet it is beginning to sound like a looped tape. A team at this level cannot defend this generously and expect to get away with it. David Luiz is simply too error-prone. Gary Cahill found his first Champions League night a torrid experience. Raul Meireles was booked, substituted and will be ineligible for the return leg. It needed a brilliant goal-line clearance from Ashley Cole to prevent Christian Maggio from making the ordeal even more harrowing. When Napoli tot up their chances they are entitled to feel they could have built an unassailable lead.
Walter Mazzarri's players always seemed that little bit quicker to the ball. Chelsea defensive line was rarely straight. Most of all they lacked the knowhow of a top European side. A better team would surely have made more of the good fortune when Paolo Cannavaro went to intercept Daniel Sturridge's low cross and managed only to slice the ball into Mata's path for the Spaniard to score the first goal. Instead Chelsea capitulated.
Mata's cool finish was the one moment of the night when this chaotic, throbbing stadium was thrust into silence. Yet Chelsea did not have the wit to keep the ball. These were moments, undoubtedly, when they badly missed John Terry's experience. What Chelsea really needed was some leadership but Frank Lampard started on the bench and Cole was also a substitute, despite declaring himself fit. Between them these three players have made more than 1,300 appearances for the club.
The warning signs had been in place even before Cannavaro's mistake.
Lavezzi, Cavani and Hamsik were elusive opponents, a constant menace in the way they interchanged positions and played the ball quickly and penetratively. Petr Cech had already made three splendid saves before Lavezzi advanced into a shooting position, with Meireles not closing him down, and tried his luck from 25 yards. His shot had enough pace and curl to beat Cech and from that point the game was turned upside down. The volume increased again and, with the final attack of the half, Cannavaro crossed from the right into a penalty area heavily populated with Napoli players. Cavani had managed to get in front of Branislav Ivanovic at the far post and the decisive touch came off his shoulder.
Chelsea will reflect on the moment, at 1-0, when David Luiz sent an unchallenged header over the crossbar from Mata's corner but they would be wrong to think the final score was an inaccurate reflection of the play. Cavani missed a decent chance to extend the lead early in the second half, quickly followed by Lavezzi wasting an even better opportunity.
The sixth-placed side in Serie A were playing with great purpose but the galling thing for Chelsea was that they contributed so heavily to their own downfall. What should have been a routine clearance from David Luiz after 65 minutes came back off Cavani and, when Cech raced out to try to clear the danger, the goalkeeper was caught in no-man's-land. Cavani squared his pass for Lavezzi to fire into an exposed net and afterwards nobody really took Villas-Boas too seriously when he tried to argue his team had concocted the same number of scoring chances.