This is the view of Barcelona legend Ronaldinho.
Barcelona secured a 1-0 victory in the first leg at Old Trafford, however, United frustrated Barca for long spells in Manchester.
Ronaldinho believes the first goal will be crucial to how the game will play out in the return tie.
He thinks if United can get the opening goal at the Nou Camp then Barca’s fans will start to get nervous and could turn on their own side.
“The truth is that Barcelona should have finished the game in Manchester,” the Brazilian said to the Daily Star.
“But at just 1-0 Manchester United will still believe that are in this tie.
“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.”
United boss Ole Gunnar Solskjaer believes his side can reproduce the magic that saw them overturn a two-goal deficit against Paris St-Germain in the previous round.
“It will have helped us as a team,” Solskjaer said.
“We know we can turn things around.
“Barca here at the Nou Camp is a difficult prospect but that night will live with the players.
“It was not that long ago and we are one goal better off [than we were then].”
Ronaldinho, who won the Champions League with Barca back in 2005/06, is backing his former side to progress to the semi-finals.
The Catalan giants are currently leading La Liga and Barca boss Ernesto Valverde rested a number of stars, including Lionel Messi, for their league game against Huesca last Saturday.
Barcelona have not lost a game at home in Europe for 30 games and Ronaldinho is predicting a comfortable win for them.
“With the way that Barcelona have been playing and the amount of goals they have been scoring, I do expect them to progress,” he said.
“I see them even winning comfortably in Barcelona. It will all be on the first goal.”