The Spanish champions take on their English counterparts in the final of Europe’s premier club competition at Wembley in London on Saturday evening in a repeat finale of 2008-09.
Two years ago Barcelona defeated United 2-0 in Rome, thanks to goals from Samuel Eto'o and Messi, and will once again start this year’s final as favourites.
Messi, though, believes that it will be unfair to write off Sir Alex Ferguson’s side, who won a record 19th title this campaign.
"They won the English championship and they are in the Champions League final,” the Argentina international said in a press conference on Monday. “They can't be very weak if they have achieved all that.
"It is a final, it is a special match given what is at stake. The good memory of Rome is behind us. This will be a totally different match. We are facing a very strong adversary, which has very good players."
He added: "I knew in pre-season we had a great team and had the chance to win everything again. I always feel we're going to win with the team we have."
Messi recalled his goal against United in the 2009 final, saying: "It was a beautiful feeling for what the goal meant to me, to the team and the fans. It's a beautiful memory.
"I hope to score again at Wembley. I hope to strike again but if not I just want to beat United and lift the Cup. I play every game as if it is my last, as if it is a final. I never want to lose."
He added: "I don't know how Manchester will play. They're a team used to having the ball and create chances. But we will try and play our game which is to keep the ball and create chances.
"The beautiful memories of Rome are already behind us. It's going to be a totally different game to two years ago.
"We are facing a great rival with very good players. It's going to be a finely-balanced match."
Messi also praised United striker Chicharito after an impressive first season at Old Trafford.
"Chicharito's become a hugely important player for United where he's scored so many important goals," he said.
"I played against him in the World Cup and he also scored against us.
"He's a very fast player, a very intelligent striker and adapted well at a great club like United in a short space of time."