In fairness to Joe Hart, he was not the only goalkeeper at Stamford Bridge who will wince when he sees the replays. Petr Cech got his angles badly wrong for Sergio Agüero's goal. He was not anticipating the quickness of the shot, or the upward trajectory of the ball. Cech seemed to be anticipating a shot to the opposite side and his body weight was leaning the wrong way. Watch closely and you can see his left leg buckle slightly as Agüero let fly.

There is, however, a good reason why all the scrutiny has been on Hart since Manchester City's defeat at Chelsea and it is not solely because his mistake, coming in the last seconds, was the decisive one. It is the deja vu that accompanies it, the sense of it being another one to heap on to the list, and the wonderment that there always is in sport when someone who once excelled has started to unravel publicly.

If we are going to be generous, it was the sixth time this season Hart has been at fault when the opposition have scored. Others would contest the numbers are even worse and that there was a degree of responsibility for Romelu Lukaku's goal for Everton and maybe one or two others. But let's cut him some slack and call it half a dozen, starting off with the one he let through his gloves when James Morrison took aim for Scotland against England. Fraizer Campbell's first in the 3-2 defeat at Cardiff is another one. There was Andreas Weimann's winner for Aston Villa. Plus, of course, two more against Bayern Munich. Not even three months have gone and, however it is dressed up, it is a wretchedly long list.

Then go back to last season and the first part of Hart's annus horribilis and it is not too difficult to bring the number into double figures. His worst performance of the lot, vulnerable from first minute to the last, was probably England's 4-2 defeat against Sweden last November. By then, Hart had already misjudged a corner, just as he did against Cardiff, for Kamil Glik to head in a soft equaliser in England's 1-1 draw in Poland. At other times, goals have originated from simple handling errors: Adam Johnson at Sunderland; Andy Carroll against West Ham and Steven Davis (from a Rickie Lambert shot) at Southampton. Others, too; in total, 15 or so goals in the last season and a bit can be aligned, to varying degrees, to Hart's goalkeeping.

Totting them up is the easy thing. Understanding what has gone wrong, and why it has reached this stage, from the odd aberration here and there to a full-blown slump, is far more difficult. At times it has been his decision-making, when to leave his penalty area or when to stay, encapsulated in that moment at Stamford Bridge when Gary Neville, in his commentary position with Sky, watched Fernando Torres slip the winner into an empty net and summed it up with three words: "Sunday league football."

There have been other misguided, or misjudged, dashes outside the penalty area, such as the one for Zlatan Ibrahimovic's fourth in Stockholm and Weimann's goal at Villa, bringing to mind a particularly senseless 30-yard sprint, for a ball that was never his, against Blackburn Rovers at a time in his City career when these mistakes felt like a one-off.

In other moments, Hart has lost control of the penalty area from routine set pieces. He has been beaten inside his near post – the goalkeeper's pet hate – more times than he will care to remember. A couple of seasons ago, it felt like nit-picking to point out he had a slight habit of beating out shots into areas of the penalty box where the opponents could latch on to the rebound, rather than turning them away to the side of his goal. Otherwise, he was pretty much flawless back then, fully deserving his recognition as one of the finest goalkeepers in Europe. Now, to analyse his performance fairly, it is futile trying to skirt over what is so blindingly obvious. Hart has interspersed some good, occasionally excellent, performances with being a threat to his own team-mates and, when it comes to identifying what has gone wrong, there is really no simple answer. A bit of everything, probably.

Hart's own belief, apparently, is that perhaps all that time working with foreign goalkeeping coaches has meant not concentrating fully on what could be described as traditional English goalkeeping, such as dealing with corners or free-kicks into the penalty area.

When Roberto Mancini was manager, another Italian, Massimo Battara, came with him from Internazionale to work with Hart. Then Mancini left and Hart wanted City to bring in Dave Watson, his goalkeeping coach with England. Hart had previously worked with Watson while on loan at Birmingham in 2009-10 and these relationships are important, as anyone who noted Pepe Reina's regression once Xavi Valero left Liverpool will be able to testify. Instead, Manuel Pellegrini brought in Xabier Mancisidor, a Spaniard who had previously worked with him at Málaga and Real Madrid.

The souring of Hart's relationship with Mancini cannot have helped either. Did Mancini want to get rid of Hart at the end of the season? David Platt, formerly Mancini's assistant, says his comments about liking Asmir Begovic at Stoke City have been taken out of context. But Mancini had discussed moving Hart out. It was not only his form that aggrieved the manager, but his manner behind the scenes, to the point where he looked at him with almost permanent suspicion. Their relationship was shot to pieces.

Mancini's judgment was not always clear in his final year, it has to be said, but his impression of Hart does fall in line with the increasingly common view, regularly put forward by Roy Keane, that maybe the goalkeeper has been guilty of blurring the lines when it comes to confidence and cockiness.

A lot of the journalists on Hart's patch would certainly understand that point, as the courteous and always impressive talker of pre-2012 has brought an edge of arrogance, often bordering on surliness, into his public dealings.

However, a more flattering way of putting it might be that Hart has actually suffered, as odd as it sounds, from his own excellence. By being so accomplished in City's title-winning season, perhaps he reached a comfort zone, with no real danger of losing his place, either for his club or national side. In previous parts of his career, Hart always had to fight for everything (Mark Hughes had sent him to Birmingham on the basis he made good, but not excellent, saves). It was when everything was in place and he was being talked about as a rival to Neuer, Buffon and Casillas that he started to fall away.

Almost certainly, there will be more to it as well. David de Gea's difficult times at Manchester United are well documented. Yet nobody would have known until that remarkable interview with Eric Steele, formerly United's goalkeeping coach, that he had a habit of "training poorly", was "lazy" when it came to learning English and had a poor diet, eating "too many tacos". Steele talked about De Gea with great affection but described his first six months as "horrendous".

Yet Hart is now behind De Gea and flailing so regularly it could hardly come as a shock if Pellegrini uses the Capital One Cup tie at Newcastle United on Wednesday as a chance to examine Costel Pantilimon's credentials on a more regular basis.

Whether Pantilimon is good enough to replace Hart permanently is another matter. Otherwise, the choice is Richard Wright, now 34 and signed as a free agent after a brief spell at Preston North End. Before that, the only other competition was Stuart Taylor, whose career can be summed up by his total of 67 starts during 16 years in the game. Hart, to put it bluntly, has had it too easy, too long.

What happens next depends on Pellegrini's tolerance levels. His manner after the Chelsea defeat, castigating City's habit of conceding "stupid goals" but refusing to talk about Hart specifically, could be taken either one of two ways. On one hand, it showed that the Chilean would never criticise his own players in the way his predecessor once did. On the other, it seemed as though he had run out of ways to defend his man.

Roy Hodgson will know how it feels, looking ahead to the next two England friendlies and wondering whether it is time to give someone else a go.