Saturday's FA Cup quarter-final at Old Trafford against Fulham, who will have no fear having won 3-1 there in the league earlier this season, marks the start of a run of three games in eight days that could make or break United's campaign.

On Tuesday, United host Porto in the return leg of their Champions League first knockout phase tie attempting to overturn a 2-1 defeat without captain Roy Keane, sent off in the first leg last week.

The following Sunday they cross town to face rivals Manchester City, by which time their deficit to premier league leaders Arsenal, who visit Blackburn Rovers the day before, could be a gaping 12 points.

Winless in their last three matches, with Dutch striker Ruud van Nistelrooy out of sorts and the suspensions of Keane and defenders Gary Neville and Rio Ferdinand biting hard, United's on-field problems are mounting.

The manager's row over a racehorse with United shareholder John Magnier is also rumbling on in the background and after nearly two decades in charge, Ferguson's proven ability to turn adversity to United's advantage is set to be tested severely.

By selling crowd favourite David Beckham for $40 million to Real Madrid before the season started, Ferguson made a potential rod for his own back. Judging by some of the emailed comments on United's own website this week, the patience of their fans is already wearing thin.

"We've missed Beckham massively", "Where is the killer instinct?", "United are playing the worst football I've seen in years", "What a joke? Absolutely pathetic!" were some of the comments posted following the 1-1 draw at Fulham on Saturday.

DEEP ROOTS

The fans' vitriol is surprising but it stems from deep roots.

United's late run to snatch the title from Arsenal last season masked a number of deficiencies in the squad which remain to be addressed.

Their lack of defensive cover has been exposed by Rio Ferdinand's controversial eight-month ban for a missed dope test and the poor discipline that led to the suspensions currently applying to Gary Neville and Keane.

An untimely injury to Mikael Silvestre and the uncertain form of Wes Brown following his long injury lay-off have left the heart of United's defence badly weakened just as the season builds to a climax.

Beckham was bound to be missed and new signings like David Bellion, Cristiano Ronaldo, Kleberson and Eric Djemba-Djemba were, by Ferguson's own admission, bought with United's future, not their present, in mind.

Some fans argue Ferguson also failed properly to replace his assistant Carlos Queiroz, who left for Real Madrid with Beckham.

Queiroz himself was a replacement for Steve McClaren whose highly effective partnership with Ferguson ended when he became Middlesbrough manager in June 2001.

Last Sunday McClaren inspired Middlesbrough to win the first trophy in their 128-year history by beating Bolton Wanderers in the League Cup final.

That silverware drought certainly puts United's troubles in perspective but it is a measure of the standard Ferguson has set himself that another fruitless campaign could leave his job in jeopardy.