André Villas-Boas's diagnosis was brutally honest. Chelsea have not written off a title challenge just yet, their faith fuelled by the stumbles being endured by some of those above them, but there is an acceptance that, up to now, they have hindered themselves. They have not been what they once were. "What other teams see in our run of results is inconsistency," said the Portuguese. "That has been our cancer this season. We are up and down, perhaps not in terms of performances but definitely in terms of results."

This campaign has been littered with false dawns. Whenever Chelsea have threatened to inject real momentum and conviction into their challenge, they have stuttered. Their toils last month summed up the season to date: a victory over Manchester City that should have inspired was followed by a wasteful draw at Wigan; an encouraging performance at Tottenham Hotspur was undermined when Fulham and Aston Villa of lower mid-table were not subsequently beaten. Too many opportunities have been passed up. That the leaders are only 11 points clear of Chelsea feels surprising.

A visit of Sunderland, even a side whose confidence is bolstered by the Martin O'Neill effect, might normally represent a chance for the London club to reimpose themselves, but it now has the propensity to induce trepidation. It is the vulnerability at Stamford Bridge, an arena that used to feel impregnable, that has been most striking over the last 13 months and the Wearsiders, then under Steve Bruce, were the first to expose that fragility. Back in November 2010 they had arrived with memories of shipping seven in south-west London the previous season still fresh, only to deflate the locals with a 3-0 success that sent Chelsea hurtling head-first into Carlo Ancelotti's often quoted "bad moment".

That proved the first of five home defeats suffered in 23 matches, the last of which was endured against Aston Villa on New Year's Eve. It is a sloppy record made all the more damning by the fact that only three league games had been surrendered in the previous 120, dating back to José Mourinho's arrival in 2004. Dominance at home is so often used as a springboard, yet Chelsea have imploded too often where once they only thrived. That has hurt Villas-Boas most of all. Had Fulham and Villa been beaten, as the manager had convinced himself they would be in the wake of a series of away draws, Chelsea would be only six points from the top today.

There is a theme that runs through each of the three losses suffered at home this term; the hosts have ended up chasing victories in matches they believed had swung their way only to suffer on the break in the closing stages. Against Liverpool and Arsenal, Chelsea had clawed back deficits and sensed their opponents were wounded.

Villa might have been content to settle for the point they held, only to bite twice on the counter. Certainly, each of the visitors benefited from the home side's willingness to over-commit, leaving a defence which is not as watertight as it was exposed and eventually exploited.

The ongoing pursuit of Gary Cahill, who should provide more resilience at the back, is a reflection of uncharacteristic weaknesses this season, though the manager's defence of David Luiz and José Bosingwa, two perceived as the most error-prone, is unflinching.

"David is going to be one of the greatest central defenders in the world," said Villas-Boas. "Why? Because of his characteristics: technical ability, anticipation and speed. I think he's played fantastically well here, but sometimes people have misconceptions and a player has to carry that stamp for the rest of his life." The observations of television pundits continue to infuriate the Portuguese.

"If everybody had perfect games, media pundits would have no jobs as there'd be no one to criticise. They'd have to find jobs in professional football, which is a little bit harder."

Yet it is not only in defence that Chelsea have been found wanting. For long periods against Fulham and Villa, the home side had appeared aimless and devoid of zest and imagination, anxiety spreading from the stands to the players on the pitch. Even Portsmouth of the Championship held them until the interval on Sunday. It is on occasions such as these that the failure to secure a Luka Modric-type playmaker who can infiltrate mass defence and illuminate an occasion is felt so keenly. Games need not be chased if opponents are unpicked early and overrun.

Instead, Chelsea have been left all the more reliant upon Ramires's energy through the middle and Daniel Sturridge's eagerness on the flank, while forever trying to bring the intelligent Juan Mata into play. Those are fine weapons to have, but too often all three need to click for the team to prosper.

The manager will hope the late win at Wolves can at least inject confidence into the ranks to help kickstart a run of victories that will thrust the west London club closer to contention.

"We have a job to do to prove people are wrong to write us off," said Villas-Boas. "We have to do that with actions on the pitch, not with words." Starting against Sunderland he must pluck consistency from the ether if Chelsea are to trouble the leaders.