Defending champions United are planning to complete a quadruple of trophies by defeating the Catalans, who will need to put up an uphill struggle to be reigned European champions.
"It's not easy to beat United," Reina told Spanish newspaper Sport. "The key is to approach them head on.
"You can't go out there with a heavy heart nor with fear, because if you do they'll end up killing you. That's what we did."
The Red Devils are enjoying one of their finest seasons, having retained the Premier League title, won the Club World Cup and Carling Cup.
However, Reina reckons Barcelona could defeat the Premiership giants should they maintain ball possession.
"United suffer when they don't have the ball, like Barca. To beat them you have to dominate possession and play without fear, with nerve," he said.
"But I'm not the one to say it. Guardiola knows better than anyone what to do and how."
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Reina, who started his career at Barcelona, conceded that he would like to rejoin Nou Camp at some point in the future, assuring in the same breath he has no intention to leave Liverpool in the meantime.
"Any footballer would love to play at Barca," revealed the 26-year-old.
"Knowing it as my home, it would really excite me, but my place now is at Liverpool, where I feel very comfortable.
"Moreover, Barca already have a great goalkeeper. Victor Valdes has had a great season and if Barca have reached so far, also in the Champions League, it's because of his good work."