WHAT a difference a week makes. Seven days ago, Manchester United stood on the brink of their tenth Premier League title and a place in the final of the Champions League. Two games later, and they are close to collapse.

Insipid in the Nou Camp and imploding at Stamford Bridge, Sir Alex Ferguson's side are hardly giving off the air of double-winners-in-waiting.

While neither of the club's last two results are terminal to their chances of finishing the season bathed in glory, their combined effect has still been to produce an air of impending crisis. Hardly the best backdrop to a Champions League semi-final second leg against Barcelona.

On the face of it, last week's first leg in Catalonia produced a decent enough outcome from a United point of view. A goalless draw means a victory this evening will guarantee a place in the final.

But the club have been here before and failed. Eight years ago, they travelled to Real Madrid as reigning champions and returned to Manchester buoyed by a goalless draw. An hour into the second leg, and Real had scored three away goals to end the tie as a contest.

At the time, Ferguson acknowledged that a failure to score in Spain had cost his side dearly, so it is telling that the Manchester United boss still felt the need to name a negative starting line-up in the Nou Camp six days ago.

Wayne Rooney on one wing, Park Ji-Sung on the other - hardly the attacking approach that has earned United deserved plaudits this season.

Perhaps Ferguson figured that Barcelona's dreadful away record in La Liga meant that an away goal at Old Trafford was likely to be beyond his opponents? With Ronaldinho injured and Thierry Henry feeling his way back to full fitness, Frank Rijkaard's side remain in a fragile state despite the presence of the likes of Samuel Eto'o and Lionel Messi.

Yet if that was Ferguson's attitude before the game, why didn't he go for the jugular and attempt to win the tie at the first time of asking?

The same question can be asked of Saturday's trip to Stamford Bridge. A swashbuckling 60 minutes from Cristiano Ronaldo, Rooney and Tevez could have ended the title race at a stroke, but instead Ferguson opted for a second-rate midfield and paid a heavy price as a result.

Undue caution has rarely been a part of his make-up in the past, so there must have been a pretty good reason for it last week. In sacrificing his attackers in order to bolster his midfield, it appears that Ferguson was attempting to protect a defence that is suddenly threatening to fall apart at the seams.

For all that Ronaldo and Rooney have hogged the headlines this season, Rio Ferdinand and Nemanja Vidic have arguably been United's two most important players for the majority of the campaign.

Had Vidic played in the Nou Camp last week, there is every chance Ferguson would have stuck to his preferred formation and concluded that defence was the best form of attack. In the Serbian's absence, however, he was suddenly scrambling around to provide a shield for a weakened back four featuring Wes Brown at centre-half.

Vidic was back at the weekend, only to disappear after 14 minutes following a clash with Didier Drogba. More shuffling ensued, culminating in the appearance of both Owen Hargreaves and John O'Shea as Ferguson desperately tried to stem the flow of Chelsea's attacking.

Having sat out training yesterday, Vidic is a major doubt for tonight's match, and for all that Rooney poses a massive attacking threat, the defender's absence would be considerably more costly than that of the England international.

Needing to fashion at least one goal to progress - unless he wants to subject his side to the lottery of a penalty shoot-out - Ferguson cannot afford to name a defensive starting line-up if Vidic is ruled out of action.

He will surely have to name Rooney, Ronaldo and Tevez if all three are available, and that will guarantee Barcelona's midfield more time and space than they enjoyed on home soil last week.

The visitors know that an away goal could prove decisive and, as a result, United's defence should expect one of their toughest examinations of the season.

They stood firm six days ago, despite Vidic's absence and the slickness of Barcelona's passing ensuring that the Spanish side enjoyed plenty of the ball. They will surely have to do so again if United are to travel to Moscow at the end of next month