For Dwight Yorke and Diego Forlan in particular, the Riverside Stadium proved a cold and forbidding place.

The booing that emanated from a section of home fans when Middlesbrough target Yorke made a substitute appearance ten minutes from time was surely as off-putting as it was perplexing for the £7m-rated striker.

Triumphant Boro manager Steve McClaren had talks with United counterpart Sir Alex Ferguson on Saturday night in an effort to finally push through a deal for Yorke, whose wage demands are understood to be a major stumbling block.

But the less-than-hospitable reception, combined with the fact that Yorke is now cup-tied, are negative factors which McClaren could have done without.

Certainly the bewilderment over the fans' adverse reaction to Yorke wasn't confined to the player himself.

"I don't know what that was about," admitted McClaren. "I can't explain it and I can't understand it."

As for Forlan, the Uruguayan striker found it necessary to travel incognito.

Having snubbed a move to Boro at the last minute eight days earlier and instead made for Old Trafford, Forlan clearly anticipated an angry response from the natives when he stepped off United's team bus on arrival at the Riverside.

With a baseball cap pulled firmly down over his eyes and his mane of blond hair tucked inside, he cut a reluctant figure to say the least.

He only went along for the ride and even waited until the stadium was empty before venturing on to the pitch to "warm-down" with his teammates. But the day was a chastening experience for the entire United party, and none more so than Sir Alex.

If he remains true to his word on his retirement plans, this was his last match in the FA Cup, the first trophy he won as United boss nearly 12 years ago.

And, after the old master had been taught a painful lesson by his one-time star pupil, he made no attempt to disguise his disappointment and disgust.

"A careless, scrappy performance," was how he summed it up. "To give that game away is ridiculous. It was down to individual errors once again - there were bad mistakes."

Perhaps the biggest mistake of all, however, was made by Sir Alex himself.

Ruud van Nistelrooy and Ryan Giggs have been nursing niggling injuries, but if they were fit enough to start on the bench and eventually perform for almost half an hour, they must have been fit enough to start the game.

The fact that they didn't, along with Yorke, hinted at an arrogant assumption on Ferguson's part that Boro were mere cannon-fodder.

For 45 minutes, there was nothing to suggest otherwise and McClaren, Ferguson's former assistant, conceded: "It looked like we were just waiting for them to the first goal. "It was a bloody long 45 minutes and it was uncomfortable in that I couldn't wait to get into the dressing room to change things."

Crucially, after Ferguson had given Boro a sporting chance with his team selection, United failed to make tell an embarrassing amount of possession in a poor first half.

Boro regrouped at the interval, McClaren withdrew ineffectual widemen Carlos Marinelli and Allan Johnston, switched to three at the heart of defence with wing-backs - and worked the oracle.

Boro were unrecognisable as they tore into United and turned the game around in amazing fashion.

Interval substitute Mark Wilson, who McClaren lured from United in the summer for £1.5m, and Dean Windass forced saves from Fabien Barthez, and Colin Cooper was only inches away with a free-kick deflected wide by a teammate.

But Paul Scholes missed United's best chance when he somehow skewed wide after Ole Gunnar Solskjaer had allowed skipper Roy Keane's centre to run across goal.

Robbie Mustoe, who as early as the 11th minute had nodded a Ronnie Wallwork header off the line, repeated the rescue act in the 67th minute.

Mikael Silvestre met Giggs' corner with a downward header, but Mustoe was stationed on the left-hand post again, this time chesting the ball away.

With the clock running down, the scoresheet blank and Yorke introduced at the expense of Solskjaer, McClaren still feared the worst.

He revealed: "I turned round to the bench and said: 'It looks set up for Dwight.' But fortunately we had the luck at the right time and it was our day."

Yet it was those "bad mistakes" to which Ferguson referred, rather than any good fortune on Boro's part, that ultimately undid United.

And not for the first time since goalkeeper Barthez and centre-back Laurent Blanc came to these shores, a French farce ensued in the champions' defence.

Five minutes remained when Boro keeper Mark Crossley launched the ball forward and a leaden-footed Blanc watched as it sailed over his head for Noel Whelan to break clear and poke his shot through the legs of Barthez.

It was Whelan's fourth goal in seven games.

Four minutes later, substitute Andy Campbell, who hadn't scored for nearly two years since netting here in a 4-3 defeat by United, unleashed scenes of wild celebration when Boro capitalised on a loose clearance from blundering Barthez.

Mustoe challenged, Whelan released Windass on the right, and from the cross Campbell headed home at the far post.

Campbell, yet to start a game this season after a serious ankle problem, had come on for thigh-injury victim Wilson, and McClaren said: "It was fortunate in one sense because it brought Andy into the game."

Given that Boro had started without seven senior players, this was a truly astounding victory for a side lying in the Premiership's bottom three.

Now the challenge is for them to reproduce results like this in the League and banish relegation worries.

"If it doesn't help the players, there has to be something wrong with them," said McClaren.

"If they don't gain self-belief and confidence from beating Manchester United, they never will.

"We were frustrated at half-time because I thought the players were showing them too much respect.

"We didn't pass the ball and we didn't compete, and it was just a case of a change in attitude and showing some passion.

"Passion is the main ingredient in a cup-tie and we certainly showed that in the second half."

So, too, in the end did an alarmingly sparse crowd - the 35,000-capacity Riverside was little more than half full.

"This is what the players can do against the best and now they have to produce that against everybody to bring the fans back," admitted McClaren. "Hopefully this result will spread the word."

They're certainly all talking about it on Teesside.