Alex Ferguson's "dream final" in Wednesday's Champions League showdown could be a nightmare for Barcelona if Manchester United smother the Spaniards and seal victory on the counter-attack. It is hardly the scenario the neutrals are hoping for but Ferguson knows the best chance of United becoming the first team to retain the title in the Champions League era is to frustrate his free-scoring opponents at every turn. They did it a year ago, shutting out Barcelona twice in a 1-0 aggregate semi-final victory, and would have taken heart from the way Chelsea kept them at bay for more than 120 minutes in the semis before Andres Iniesta's stoppage-time strike. Ferguson admires Barcelona's approach and has shown his own commitment to exciting football by regularly starting with Cristiano Ronaldo, Wayne Rooney and Dimitar Berbatov as an attacking trio. However, the vastly experienced Scot knows that great teams are built from the back and the key to unlocking your own weapons is to neutralise the opponents. "They get you on that carousel and can leave you dizzy," he said of Barcelona's mesmerising passing game. "Your concentration levels can't be allowed to falter for a second. But, with the right tactics, their game is containable." United's defence has been magnificent this season and, assuming Rio Ferdinand is fit to line up in the middle alongside Nemanja Vidic, it should be at full strength. HARD MIDFIELD It is United's hard-working midfield, though, which has helped limit the rest of the Premier League to just 24 goals against them. In the Champions League they let in only three in the group stage and four in six knockout games against Inter Milan, Porto and Arsenal. The non-stop running of Park Ji-sung and the game-reading of Michael Carrick provide a first line of defence that would make any back four look good. "The way Barcelona play, they can have the ball for a long time and you've just got to be patient, defend well and concentrate. It's quite a bit of a mental battle," said Carrick. "Against Barcelona you shut up, as Chelsea showed, and then attack them at the right time." United have perfected the art of sucking teams in then striking on the break, a tactic shown at its best when Ronaldo started and finished, after a lung-bursting 60 metre sprint, their marvellous third goal in the semi-final win at Arsenal. LOOK VULNERABLE That is exactly the sort of thing Ferguson will be looking for on Wednesday, particularly as Barcelona look vulnerable at the back after the suspension of Eric Abidal and Dani Alves and the injury that has ruled out Rafael Marquez. Manager Pep Guardiola, who rested almost all his first team for Saturday's 1-0 league defeat by Osasuna, is also sweating on the fitness of Iniesta and Thierry Henry. Like United, Barcelona have sown up their league title with time to spare and both have also claimed a domestic cup. But also like United, who suffer in comparison with Liverpool's European exploits, the Spaniards desperately crave the European Cup that they have won just twice in comparison with Real Madrid's nine. They will go about that task with patience in Rome, backing their approach and looking for the relentless probing of Iniesta and Xavi to eventually prize an opening that Henry, Messi or Samuel Eto'o will be able to seize upon. Eto'o, who did play on Saturday in a bid to find his 30th league goal of the campaign, summed up the team's approach. "We've scored 150 goals this season," he said. "For a striker there is no better place to be than this Barcelona."