Sir Alex Ferguson owes Barcelona a debt of gratitude, and not because it was their stadium that provided the scene for the greatest night of his managerial career. Sure, the last-gasp 2-1 victory over Bayern Munich at the Nou Camp in May 1999 will always rank as one of the most important moments in Manchester United's history, but would the club have been celebrating a second European Cup had they not been so humbled by the Barcelona of Hristo Stoitchkov and Romário 4 years earlier?

Such questions may seem immaterial, but to listen to Ferguson speak on the eve of the first leg of United's Champions League semi-final against Barcelona this evening, the 4-0 battering that his team received in Catalonia in November 1994 was no less significant in terms of shaping the club's history and his tenure as manager.

“The defeat in 1994 was a great occasion for us in the sense of realising how important it is to retain the ball,” Ferguson said yesterday. “In terms of the quality and possession of the football, Barcelona were far better than us, so it was a good lesson.”

Against a Barcelona side who love to exploit the size and width of the enormous Nou Camp pitch, Ferguson believes that United cannot afford to fall into that trap again. But he was impressed with the manner of his team's 2-0 victory over AS Roma in the first leg of their quarter-final triumph in Italy last month - when “our retention of the ball was excellent”.

The United manager is also bullish about the composition of his squad, comparing his present side favourably to the team who won the Champions League nine years ago. “You look at the form of Michael Carrick and Anderson and how they have developed, at Wayne Rooney and Cristiano Ronaldo, who are as good as the players we had in 1999, and you wouldn't have any fears,” Ferguson said. “They play without fear. They know it is a semi-final and you can't win a semi-final without showing your true mettle.”

Much like Liverpool, Barcelona's season may have been overshadowed by infighting and uncertainty over the future of Frank Rijkaard, their coach, but despite lying third in La Liga, 11 points adrift of Real Madrid, the leaders, Ferguson believes that league form may count for nothing and would not be surprised if the teams played out another 3-3 draw, as they did at the Nou Camp and Old Trafford in the group stage in 1998.

With Ronaldinho injured and Thierry Henry liable to start on the substitutes' bench, Barcelona will look to Lionel Messi to dictate the tempo, much as United will hope that Ronaldo produces the form that helped him to plunder 38 goals this season.

The United manager would put money on his man eclipsing the Argentina forward. “I am not worried about him [Messi],” Ferguson said. “As long as Ronaldo does the job he has been doing all season, we will be all right.”

United are eager to ensure, though, that they do not suffer another disappointment such as that of last year, when AC Milan knocked Ferguson's side out at the same stage.

Nemanja Vidic, the defender, blamed injuries for United's knockout blow 12 months ago and made it clear that there is to be no repeat of that defeat. “We want to play much better at this stage and go on and win the trophy,” Vidic said. “Near the end of last season we had to play with some injured players. It was nearly the same faces all the time and it was a bit hard for us. Maybe that was the reason we did not do so well in the final stages of the Champions League.”

Now, with United having more strength in depth, Vidic is more upbeat. “This season we have a bigger squad and we are in good shape going into this game,” he said.

The defender cautioned his team-mates not to become fixated on keeping Messi quiet, suggesting that focusing on the Argentina striker would allow some of Barcelona's other superstars room to punish United. “He is one of the best players in the world,” Vidic said. “Everyone knows that. But the game is not all about Messi. Like many good teams, Barcelona have many good players.

“We have a tough job but we will do our best to try and contain them and get a good result.”