The Spanish giants have gone 30 home games in Europe without losing and are strong favourites to beat United and book a place in the last four.
But Ronaldinho believes the first goal will be crucial – and that if United can get it home fans will start to get nervous.
The Brazilian superstar, who spent five seasons with Barca and helped them win the Champions League in 2006, said: “The truth is that Barcelona should have finished the game in Manchester – but at just 1-0 Manchester United will still believe that are in this tie.
“With the way that Barcelona have been playing and the amount of goals they have been scoring, I do expect them to progress.
“I see them even winning comfortably in Barcelona. It will all be on the first goal.
“If Barcelona score it then it will be a long evening for Manchester United.
“But if they (United) can score the first goal then it’s going to make the stadium nervous – and you never know what can happen then.”
“If they (United) can score the first goal then it’s going to make the stadium nervous – and you never know what can happen then”
Ronaldinho, meanwhile, has also backed Tottenham to go all the way to the final.
Spurs take on favourites Manchester City at the Etihad on Wednesday night holding a 1-0 lead from the first leg.
Pep Guardiola’s quadruple-chasing outfit are still expected to go through.
But Ronaldinho insists Mauricio Pochettino’s side could be the dark horses of the competition.
He added: “Why can’t Spurs win the Champions League?
“Because they don’t have the history in the competition – or because they have never won it before?
“Every team who has won the Champions Leauge had to win it for the first time once.
“Their result against Manchester City, who are one of the best teams and have some of the best players in the competition, showed that they can compete with the best.
“At this stage they will believe they can go all the way – and if they play as they can then maybe they will.”