The red mist descended on Roy Keane in Porto tonight as Manchester United's Champions League dream started to turn into a horrible nightmare.

With makeshift centre-half Gary Neville outstanding, Sir Alex Ferguson had seen his beleaguered defence turn in their best display in weeks to somehow cling on to a semblance of hope in a tie that threatened to run away from them at regular intervals.

But four minutes from time, just as it looked like Porto's fire was burning out, so the blaze erupted in Keane's soul as the Irishman needlessly trod on Porto keeper Vitor Baia as he tried to chase down a loose ball.

The contact did not appear too fierce but the intention was enough to warrant the eleventh red card of Keane's United career.

It was a catastrophic way for United to end a contest in which the visitors had led through Quinton Fortune's early strike but, despite their defensive heroics, eventually buckled under the ceaseless weight of Porto pressure.

Benni McCarthy volleyed the hosts level before the break, then climbed highest to nod home the winner from Nuno Valente's cross 16 minutes from time.

Scanning a dressing room lacking Rio Ferdinand and Mikael Silvestre, and in the knowledge his team had leaked 11 goals in their previous five games, Ferguson knew he needed to get his defensive selection correct.

Quite how tough their examination would be became evident inside the opening minutes as a combination of slick Porto passing and speed off the ball had the visitors spinning.

Wes Brown, who had won the battle of the out-of-form centre-halves with John O'Shea, made a timely interception to deny Paulo Ferreira after just five minutes as the wing-back tried to exploit the room which opened up behind Fortune at regular intervals.

Neville's presence had a galvanising effect on Brown, the outspoken England international throwing his body at anything that threatened the United goal in those frenzied opening stages.

Yet, just as Nicky Butt and Keane looked set to buckle in an overworked midfield engine room, United advanced to the other end of the field for virtually the first time in the match and scored.

Many pundits have long since concluded that Baia's impressive reputation is gained mainly on myth and the theory was proved again as the Portuguese international keeper spilled a long-range Paul Scholes effort, which admittedly was going pretty quickly, and Fortune beat Ruud van Nistelrooy to the tap-in.

It was a fine personal recovery from the South African, who endured a miserable afternoon against Leeds last weekend, although he did not know too much about his next meaningful intervention as Pedro Mendes' shot smashed into his face as he stood guarding the far post at a Porto corner.

The visitors had only just survived one major scare when the excellent Russian midfielder Dmitri Alenitchev skipped inside three tacklers after taking Deco's brilliant lofted pass inside the United box.

It was pretty needless given he had originally been given plenty of room to shoot anyway and the error of his ways was proved when he eventually blasted his shot into Neville's body.

After two great escapes, it proved to be third time unlucky for United, whose rearguard finally cracked a minute short of the half hour.

Mendes was the instigator this time, whipping in a right-wing cross that offered McCarthy enough room to swivel into position and arrow a volley into the corner of Howard's net.

Fortune's eventful night took a turn for the worst when he charged into the back of Carlos Alberto, a 19-year-old who shares not only the name of a legendary Brazilian but, on this evidence, also some of his talent.

There was certainly only one winner when, giving his young opponent only half a yard of space close to the dead-ball line, Fortune found himself trying to tackle the thin air Alberto had just danced through.

Deco should have made more of his young team-mate's mesmeric burst but disappointingly for the home side, blazed the cut-back over.

It took United 20 minutes of the second half to launch their first meaningful attack and Giggs wasted it when he opted to try and release Saha rather than the much better placed Scholes to his right.

The game soon resumed its previous pattern, with McCarthy and Alberto both going close before the South African leapt highest to power Nuno Valente's cross in off the underside of the crossbar.

United were lucky it proved to be the end of the scoring. Keane's moment of madness ensured it was not the end of the incident.

Teams:

FC Porto Vitor Baia, Paulo Ferreira, Jorge Costa, Ricardo Carvalho, Nuno Valente, Pedro Mendes, Maniche, Deco, Alenitchev (Jankauskas 63), Carlos Alberto (Ricardo Fernandes 75), McCarthy (Bruno Moraes 83).

Subs Not Used: Nuno, Pedro Emanuel, Ricardo Costa, Bosingwa.

Goals: McCarthy 29, 78.

Man Utd Howard, Phil Neville (O'Shea 70), Gary Neville, Brown, Fortune, Keane, Butt, Scholes, Saha (Ronaldo 76), Giggs, van Nistelrooy.

Subs Not Used: Carroll, Bellion, Kleberson, Djemba-Djemba, Forlan.

Sent Off: Keane (87).

Booked: Fortune, Butt.

Goals: Fortune 14.

Att: 49,977

Ref: Herbert Fandel (Germany).