Sir Alex Ferguson admits Manchester United's defensive resolve has collapsed since their clean-sheet record was brought to an end by Blackburn.
It is hard to comprehend how a team that can set a new Premier League best of 14 successive clean sheets should suddenly appear so vulnerable.
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 Old Trafford hammering by Liverpool on March 14.
But Ferguson believes the actual malaise dates back a further three weeks, when Blackburn were distinctly unlucky to suffer a 2-1 defeat to the Red Devils, having outplayed the world club champions for long spells.
What Sam Allardyce's side did manage to do was score, through Roque Santa Cruz. It robbed United of an in-built need to concentrate, and they have never recovered.
"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 last 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 last night's 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.
Little wonder it seems like Wayne Rooney is carrying the fight on his own, with occasional flashes of genius from Cristiano Ronaldo, Carlos Tevez and Federico Macheda.
Indeed, given the importance of next week's second leg and the need to retain an element of freshness for Portugal, it would be no surprise if Macheda was handed his first start on Saturday, when the Premier League leaders head to relegation-threatened Sunderland.
O'Shea is set to partner Vidic in defence as United attempt to subdue Kenwyne Jones rather more effectively than they managed with Aston Villa's John Carew on Sunday.
However, the problem of fatigue goes far deeper.
"We are suffering," said Ferguson.
"Park Ji-sung is the one player who never misses a beat in terms of his movement and endurance work in a game.
"He has such natural stamina. He was well off the pace last night and looked tired."
Park's World Cup commitment to South Korea could not have helped, one of the reasons Ferguson felt he was placed in an unfair position last weekend when he was forced to choose between facing Villa on Saturday lunchtime with no preparation, or Sunday afternoon knowing aching limbs could drain his team last night.
Only the foolhardy would write off United's chances of overcoming Porto in the second leg next week.
But Ferguson also knows he must look beyond the task of ensuring United become the first English team to beat the Portuguese champions on their own soil after 11 previously failed attempts.
For, if United do fall, the Scot will have a major job on his hands trying to raise spirits in a squad that sees the finishing line too far away to simply drop across. And hungry predators from Merseyside - Rafael Benitez in the Premier League and David Moyes in the FA Cup - are lying in wait to pounce themselves.
well, it is the 7pts ahead with one game in hand, and two trophies in the bag and I bet SAF would have taken his foot off the pedal that started all this..
now, to lash that whip and focus..