Luiz Felipe Scolari has gone to great lengths in the past few days to insist that all is well in the Chelsea dressing room. We will know the truth in the coming weeks as the Blues' challenge for major honours enters a critical period. The next four away games - against Manchester United, Southend, Liverpool and Aston Villa - will define their domestic season.

Scolari has already acknowledged that results at Stamford Bridge "are not good" but he has been spared embarrassment by his team's record-breaking form on the road this term.

But even that is beginning to look a bit rockier than usual. Dropped points at Everton and Fulham generated a lot of dressing-room soul searching.

Now Old Trafford looms menacingly on Sunday. There is a perception that all is not as it should be at Stamford Bridge after Chelsea have suddenly hit a streaky patch.

Ask Sir Alex Ferguson. He's had plenty of experience of streaky patches and, each time, he's emerged triumphant. The big test for Scolari comes at Old Trafford when, to be frank, defeat is unthinkable. If pacemakers Liverpool win at Stoke and Ferguson outwits Scolari, Chelsea will be six points adrift of the leaders with United just one point behind them with two games in hand.

A few days later Chelsea face the prospect of a difficult FA Cup replay at Southend - an assignment that many insist would not have been necessary in the days of Jose Mourinho. The 'Special One' made Stamford Bridge a fortress from which no team ranked 13th in League One could possibly hope to emerge with a 1-1 draw.

But Southend did just that last week, exposing a vulnerability that Ferguson is sure to exploit on Sunday.

The United manager will have noticed that both Fulham and Southend capitalised on poor marking at set-pieces and Scolari has devoted training time to tightening their defensive strategy in the last few days.

Of course Chelsea have missed the injured Ricardo Carvalho and the suspended John Terry and the fact that this formidable partnership should be back at Old Trafford will significantly improve Chelsea's security.

Terry is a massive influence and his return should help Scolari restore confidence and dispel the gloom that threatens to settle over Stamford Bridge where Chelsea have won just four of 10 Premier League games this season.

The England defender acknowledged the problems when he said: "We need to be more consistent and we need to be together more."

This team spirit and shared sense of purpose took root under Mourinho and underpinned the success he brought to Stamford Bridge. Scolari is adamant that Chelsea retain the same unity and it is true that, apart from Deco and Jose Bosingwa, the Brazilian coach is working with the same group of players.

But some are now questioning whether Scolari has the support of all the players because Chelsea appear to have accepted defeat too readily. Frank Lampard admitted recently that he and some of his team-mates had been below their best at times and he made a point of urging fans to have patience with Deco, who has gone off the boil.

Love them or hate them, you have to respect the enduring excellence of United, the defending champions bidding for an 11th Premier League title under Ferguson.

With attacking talents like Cristiano Ronaldo, Dimitar Berbatov, Wayne Rooney and Carlos Tevez at his disposal, Ferguson has the firepower and the experience to sustain yet another successful challenge for the title.

But, as Scolari has found, it doesn't always go to plan. Since hitting five past Stoke in mid-November, United have struggled for goals in the Premier League, scoring just four in a run of six undefeated matches.