On Sunday afternoon there may well be a subdued atmosphere at Old Trafford. It is, as the joke goes, a matchday after all. Yet the ground may be even more muted than usual during the match against Swansea. If Papiss Cissé adds to his Le Tissier-esque portfolio of goals and Newcastle avoid defeat against Manchester City earlier in the day, Old Trafford will have the collective wide-eyed glee usually reserved for those who have been given a second chance at life. If City win, however, the title will almost certainly be gone.

A sombre final home match may be an appropriate end to a season in which United have shown rare vulnerability and insecurity on their own patch. Ostensibly a record of 10 points dropped in 18 league games is fine, but it does not compare favourably to United's record last season (two points dropped from 19 games) or City's this (two dropped from 18 games). The manner and timing of United losing points has been especially damaging, culminating in the surreal and possibly decisive 4-4 draw with Everton a fortnight ago.

The addition of European and domestic cup games shines a harsher statistical light on United's home form. Last season their record was P29 W26 D3 L0; this year it has been P24 W15 D4 L5. The number of goals they have conceded both in the league (19) and all competitions (31) is the highest since the 1970s.

The season started with two delirious eviscerations of Spurs and Arsenal; since then even the big victories (5-0 against Wigan, 4-0 over Villa) have been peculiarly joyless, even boring. Yet overall, purely in terms of results and efficiency, this has been one of United's proudest seasons. The ability to challenge for the title despite Glazernomics, Manchester City's clout and probably the most debilitating injury list at Old Trafford since 1997-98, attest to Sir Alex Ferguson's staggering capacity to manipulate resources. Yet when United have been bad, particularly at home, there has been a frequent need to delve into the record books and the memory bank.

The 6-1 defeat to Manchester City was their worst home defeat since 1955, an already seismic result that will become even more significant should the title be decided on goal difference. Had United lost that game 2-1, they would be above City on goals scored. Even City did not outplay United quite like Athletic Bilbao – their Europa League win was probably the biggest chasing United have received at home since they somehow beat a majestic Red Star Belgrade side 1-0 in 1991-92. And if Blackburn go down, they will be the first relegated side to have won at Old Trafford since 1976-77.

After the 6-1 defeat, Ferguson said United "just have to forget it as though it never happened". The apparently freakish nature of so many home games this season – the defeats and also the high-scoring draws with Everton and Basel – offer a certain solace. But too many one-offs suggest something fundamental is off.

In this case, it's the defence. United will rightly point to the extended absence of Nemanja Vidic, probably the world's best defender. He missed all five home defeats this season. If suggestions he was omitted from the 6-1 for disciplinary reasons are true, it was a costly assertion of authority.

Yet there are times when even Vidic would have struggled, such has been the chasm in front of United's defence. The absence of Darren Fletcher has been of obvious significance, as has that of Michael Carrick in certain games. Carrick missed four of the five losses; in the fifth, against Blackburn, he was an emergency centre-half rather than a screen in front of the back four.

Ferguson's decision to favour two forwards has sometimes contributed to the midfield vulnerability. The team have not had much help from the stands either. The atmosphere at Old Trafford occasionally feels like a scientific project to see if 70,000 people can immaculately observe a period of 90 minutes' silence. A number of supporters seem to have confused the theatre with Theatre of Dreams, and a nadir was reached against Blackburn, when the crowd only really summoned any enthusiasm to sing Happy Birthday to Ferguson. United's decision to introduce a singing section suggests a recognition of the problem, if not much of a clue how to resolve it.

Most relevant of all, perhaps, some opposing teams have started to scrutinise the emperor's attire more closely. Many still roll over and accept their fate, or accept their fate after Ashley Young has rolled over, but others are less defensive than was once the case. To see a side as intrepid as Bilbao go for it was not surprising; to see a similar approach from Everton, usually so compliant at Old Trafford, certainly was. "We came here to try and win the game," said David Moyes. "We really did."

In the past, to attack United on their turf was to raise hell. When a vibrant West Ham side had the temerity to take an early lead in 1999-2000, they were destroyed 7-1. Three seasons ago Spurs were 2-0 up at half time and lost 5-2. This season United have gone behind in eight games at Old Trafford. They have won none of them.