Johan Cruyff believes Barcelona must eradicate the errors which have blighted their Primera Liga season if they are to have any chance of beating Manchester United in the Champions League semi-finals.

Having slipped well off the pace in the league in recent weeks, the Catalans now host United on Wednesday in the first leg of the last-four tie and Cruyff warns they cannot make mistakes.

"Barcelona will have to do things well. If there aren't any stupidities, the tie is there to be won," said Cruyff, who achieved legendary status after hugely successful spells as a player and a coach at the Nou Camp.

Frank Rijkaard's side have not won any of their last four league games and are now 11 points adrift of arch rivals Real Madrid in the race for the title.

However, Cruyff hopes the Catalans can rediscover their best form in Europe.

"If Barcelona play like they know how, they have a chance of getting to Moscow (for the Champions League final next month).

"Even though they are stumbling in the league and have literally thrown away the title, my thoughts don't change. Europe is another story, another competition. There is a different mentality and I believe it will be an even tie."

Cruyff led Barcelona to their first European Cup success in 1992 and was responsible for the hiring of current coach Rijkaard.

Barca also achieved European glory in 2006 but have not won another major trophy since beating Arsenal in Paris two years ago.

The Catalans led the Primera Liga for much of last season but were eventually overhauled by Real Madrid, and they look almost certain to lose out to their bitter rivals in the league again this term.

Press and fans have been hugely critical of the team in recent weeks but centre-back Rafael Marquez says the club does not deserve it.

"There is a lack of support and the treatment has been unfair. I believe we marked an era and made history. It's true we are not happy with these last two years, but people forget that we won a European title," he said.

Marquez believes the team's much-maligned defence could hold the key to victory against Manchester United.

"Not letting in a goal is key, although we would like to go to England with a lead," he said.

The Mexico defender says Sir Alex Ferguson's side are favourites but believes Barcelona can advance to a second final in three years.

"They are very strong offensively and are the favourites but if we keep the ball and are compact in attack and defence, we can win," he said.

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