Vidic immediately signalled he was in trouble after falling awkwardly following a tangle of legs with Marco Streller during the first-half of the shock 2-1 defeat.

Sir Alex Ferguson said after the game his captain had suffered ligament damage, but hoped the damage was not too serious.

However, speaking to the Italian website www.calciomercato.com, Vidic's agent Silvano Martina delivered some devastating news.

"Unfortunately he has suffered a ruptured cruciate ligament," said Martina.

In response to a follow-up question asking if the player would be out for several months, the agent said: "Unfortunately so, yes."

If the worst is confirmed by Ferguson at tomorrow's scheduled press conference to preview Saturday's Premier League encounter with Wolves, it would inevitably rule Vidic out for the remainder of the campaign.

The Serbian's return from a calf problem sustained in the opening-day win at West Brom was cited by many as the moment United's defensive difficulties began to ease.

Despite repeated suggestions the player would leave Old Trafford, Vidic remains one of the key figures in Ferguson's squad and his absence would place additional focus on the younger players such as Phil Jones and Chris Smalling, who were chided by Roy Keane in Switzerland last night.

It would certainly cause Ferguson a major headache given his other defensive stalwart, Rio Ferdinand, is rarely able to string games together due to a series of well-publicised muscular problems.

The news merely heightens the sense of disappointment felt at last night's defeat, which saw United fail to emerge from the group stage of the Champions League for the first time since 2005.

"We have to be honest and say we haven't been professional from the beginning of this competition," admitted Patrice Evra.

"I don't know why.

"Maybe one or two of us have to look in the mirror and say we can do a lot better than we have done. Everyone. And I am the first."

It means instead of pondering the potential hazards posed by a second-place finish in the Champions League, including Barcelona, Real Madrid and Bayern Munich, United will need to start planning to face a whole different set of opponents.

Romanians FC Vaslui for instance, who must do what United failed to achieve last night, and win in Switzerland, against FC Zurich, to secure qualification as Group D runners-up.

Lokomotiv Moscow, Rubin Kazan and Maccabi Haifa are three others who will provide not just an obdurate challenge to be overcome but also significant travel inconvenience.

It is not a competition that is looked on with any affection by top clubs, unlike Stoke, who have had such a fine time this season, and Fulham, who reached the final two years ago.

Yet Evra is adamant United will try to win it, although their priority has to be closing the five-point gap Manchester City have opened up at the Premier League summit.

"We are Manchester United," he said. "We won't throw the season away.

"We still have things to win this season and we have to make sure we do that.

"We need to play with heart and the fans deserve more than us throwing away the Champions League and the Premier League."

Ferguson is already steeling himself for the inevitable dissection of his squad.

He rebuffed Keane's comments with a sideswipe about the Irishman's own patchy managerial record, but there is much more to come.

Fans heading out of Switzerland today continue to be baffled by Ferguson's refusal to acknowledge any deficiency in midfield, even though he was reliant on 37-year-old Ryan Giggs and Phil Jones, almost half his age, to man the central positions last night.

As it barely seemed credible that United should win a 19th league title and reach the Champions League final last season with their midfield resources as they were, and they have lost the inspiration provided by Paul Scholes since then, the confusion will continue.

Tom Cleverley's absence has been cited as one reason for United's transformation from a free-flowing August winning machine to the present stuttering side, although as the 22-year-old has played just five senior matches for the Red Devils, it surely cannot be an accurate assessment of Ferguson's reliance on him.

Yet the hopes of signing reinforcements must have been affected by last night's defeat, which has cost United anything up to £20million given the Europa League offers nothing like the financial rewards of its more illustrious counterpart.

"Nights at Old Trafford under the lights with the Champions League music coming on is what you live for and now we have gone out early," said Rio Ferdinand.

"But bouncing back is what this club does. That's what we do as players.

"We will dust ourselves off and be ready for the next game. That is what makes this club so great.

"Although we are very disappointed, we will not dwell on what's gone on.

"We have massive games coming up and we will make sure we are in the right frame of mind to win them."