The night turned out to be a comfortable one for United, but the home fans will wonder why it took until Nelson's strike for them to play with the sort of tempo that went on to cause Benfica all sorts of problems.
Who are they? Benfica are one of Portugal's big three clubs along with Sporting Lisbon and Porto. The trio have won all but two league campaigns in Portugal; Benfica lead the way with 31 titles to Porto's 21 and Sporting's 18.
A conservative 4-5-1 formation at kick-off suggested United lacked urgency, and Nelson's fierce strike confirmed the supporters' worst fears.
Recent form: It has been a stop-start campaign for Benfica so far, but things have begun to pick up in recent weeks and they recorded a third successive win for the first time this season at the weekend with a 2-0 victory at Sporting. It was their first away win in four attempts and means they stay fourth in the table.
In fairness, United's response was immediate, Cristiano Ronaldo seeing his drilled free-kick punched away before Saha's header from the resultant corner was cleared off the line.
European record: A rich history, with the emphasis on history. Back-to-back European Cups in the early Sixties when they also reached three finals. Also lost in the showpiece match in 1988 and 1990 and more recently reached the quarter-finals last season.
From then until the end of the half, with Wayne Rooney pushed back up front in a 4-4-2 formation, United lay siege to the Benfica goal.
How did they get here? A third-placed finish in the Portuguese league led to them booking a qualifying spot, where they beat Austria Magna 4-1 on aggregate.
Giggs' precise header eased United's nerves
Ones to watch: So far this season 15 different players have scored 33 goals for the club, but it is the attacking trio of Portuguese internationals Simao Sabrosa and Nuno Gomes and Italian forward Fabrizio Miccoli who provide the real threat in attack.
Rio Ferdinand and then Paul Scholes came close to drawing United level, but a brave block from Luisao and another fine Quim save protected Benfica's lead.
Did you know? What has Fabrizio Miccoli got in common with Diego Maradona? A World Cup win under his belt? Staples in his stomach? In time maybe, but for the moment it is just a tattoo of Che Guevara that the pair boast.
Despite the increased tempo, the Old Trafford faithful were becoming audibly frustrated, but a minute into first-half stoppage time they roared their relief as United deservedly drew level, Vidic rising unmarked to head into the net from Giggs' free-kick.
They say: "Seriously, it's not crazy to talk about a possible win in Manchester. We are completely capable of winning and go to England calm but optimismic. I am convinced Benfica can do some damage to United and we want to exploit that. I have asked my players to ignore the atmosphere and only to think of winning. I'm expecting them to fight until the end."Fernando Santos, Benfica coach
United refused to let up in the second half and it soon reaped its rewards as the hosts took the lead.
BIG MATCH STATS
Ronaldo, who moments earlier had seen his penalty appeal waved away correctly after he had fallen theatrically in the box, sent in a pinpoint cross that Giggs headed home with equal precision.
Definitions of terms used:-
The goal extended Giggs' run to 12 consecutive seasons scoring in Europe's premier competition and, despite a couple of impressive solo runs from Simao, the fight appeared to have gone out of Benfica.
Champions League (CL) - only group phase matches and beyond of this competition which began in 1992-93. Champions Cup/Champions League - all matches played since it began in 1955-56 including qualification matches.European matches - all matches played in the major European tournaments (Fairs Cup, Uefa Cup, Cup Winners' Cup, Champions Cup, Champions League). Uefa Super Cups, Intertoto Cups and the old Intercontinental Cup competition are excluded.
And so it proved on 74 minutes when Saha thumped home a header from Rooney's corner.
Qualification
With half an eye on Saturday's derby match against Manchester City, Sir Alex Ferguson took the chance to take off Giggs and Scholes and the veteran pair walked off to a standing ovation.
Manchester United require at least a draw in this match to progress to the last 16. Benfica must win the match to qualify for the knockout phase of the Champions League. If Benfica fail to win, they will play in the Uefa Cup.
In truth, as soon as Vidic equalised the victory had rarely looked in doubt and the hosts went on to comfortably gain revenge over the side that had dumped them out of the competition at this stage last season.
United will finish top of Group F if they beat Benfica and Celtic fail to win in Denmark. Benfica will top the group with a win and a Celtic defeat.
Head-to-heads
Man Utd: Van der Sar, Neville, Ferdinand, Vidic, Evra (Heinze 67), Ronaldo, Carrick, Scholes (Solskjaer 78), Giggs (Fletcher 73), Saha, Rooney.Subs Not Used: Kuszczak, Brown, O'Shea, Silvestre.
Manchester United have won both of their previous home matches against Benfica, and five of six games against them, with the only exception being the 2-1 victory by Benfica which knocked United out at the group stage exactly a year ago.
Booked: Rooney, Fletcher.
Benfica have won twice on their nine previous visits to England. The Portuguese club won 1-3 after extra time at Arsenal in the 1991-92 Champions Cup and 0-2 at Liverpool in last season's Champions League.
Manchester United have won the Champions Cup/Champions League twice (1968, 1999), the Cup Winners' Cup once (1991), the European Super Cup (1991) and the Intercontinental Club Cup (1999). They are taking part in their 12th Champions League season, and are aiming to make the knock out stages for the 10th time in 11 seasons.
Booked: Ricardo Rocha.
Benfica have won the Champions Cup twice, in 1961 and 1962, and are taking part in their fourth Champions League season. Their best result in the Champions League was reaching the quarter-final stage in 1994-95 and 2005-06.
Goals: Nelson 27.
Current European form
Att: 74,955
After a 100% start in their first three matches, United have now lost two in a row and are second in Group F. No club has ever won their first three Champions League group matches and failed to qualify for the next stage.
Ref: Herbert Fandel (Germany).
Benfica are third in Group F having won two home matches and drawn 0-0 at FC Copenhagen. They have failed to score in their last three Champions League games away from home.
Player and disciplinary info
Manchester United's Paul Scholes will be suspended if booked. Benfica's Kostas Katsouranis will be banned if he receives a yellow card in this match.
Ryan Giggs is the highest scoring Champions League player in the current Manchester United squad with 21 goals. Paul Scholes needs one goal to equal Giggs and Ole Gunnar Solskjær is two shy. Red Devils's keeper Edwin van der Sar and Benfica's Leo and Quim are yet to a miss a minute of this season's Champions League campaign.
Other miscellaneous facts
Alex Ferguson's Champions League total of 120 matches is a record for the competition. Benfica could suffer their 50th defeat in the Champions Cup/Champions League.
United hold a six point lead at the top of the Premiership table, while Benfica trail leaders FC Porto by nine points and lie fourth with a game in hand in the Portuguese League.
