Ferguson: Roque strike started slump

The 10 goals United have conceded in their last four games is the same number it took 22 matches to let in prior to the 4-1 Old Trafford hammering by Liverpool. But Ferguson believes the malaise dates back a further three weeks, when Blackburn outplayed them for long spells before losing 2-1.

"We were doing very well when we weren't conceding any goals because there was a record to defend," recalled Ferguson. "Once we lost that first goal against Blackburn, the battle to try to keep the record had gone. Since then we have lost really bad goals."

Injuries and poor form have also come together at just the wrong time, a deadly combination from which, at present, there seems to be no escape.

When Rio Ferdinand returned from the Club World Cup in Japan with a back injury that kept him out for five weeks, Jonny Evans stepped brilliantly into the breach.

But now Ferdinand is sidelined again, Evans' confidence has ebbed as the rigours of playing so many games have caught up with his 21-year-old legs.

Similarly, the once reliable presence of John O'Shea, who was superb at left-back when Patrice Evra was ruled out by suspension and then injury, is struggling for form after being switched to the other full-back position and then centre-half on Tuesday night when Evans went off with a groin injury that makes him a doubt for next week's second leg with Porto.

Nemanja Vidic's recent disciplinary troubles mean he was starting only his fifth game out of nine in the 2-2 draw and the rustiness showed.

The list of senior men currently operating well below their peak is lengthy and includes Evra, plus Michael Carrick and Paul Scholes, who can normally be relied upon to keep possession but are currently giving it away with alarming regularity.

Gary Neville is only just back off the treatment table, Dimitar Berbatov, Anderson, Wes Brown and Rafael da Silva are on it, along with Ferdinand, whose groin injury has not given Ferguson any sign of clearing up.