That’s according to ESPN FC pundit Steve Nicol.
United travel to the Nou Camp tonight in an attempt to make it through to the final four for the first time since they were beaten in the final by Barca in 2011.
However, the odds are stacked against them, given that the Catalan giants have only lost to one English team on home soil in the competition.
United know a thing or two about being underdogs, however, as they overturned the odds against Paris Saint-Germain in the previous round.
Kylian Mbappe and Presnel Kimpembe scored at Old Trafford in a 2-0 home leg win for the Parisians, before Rashford capped off an incredible comeback at the Parc des Princes in stoppage time in the return tie.
United won 3-1 in the French capital to go through on away goals, and Nicol warned that they will have to play with the same intensity to have any hope against Lionel Messi and co.
“I’ll be shocked if they don’t go about it the same way they did against PSG,” he said.
“That’s by playing Marcus Rashford and Romelu Lukaku up high, trying to put pressure on Barcelona.
“The one thing that United have to do is every sniff of an opportunity they get, they have to take it.
“They got three at PSG, and took every one of them. They’ll have to do that again.
“That’s without letting Barcelona hit the back of the net.
“If they thought PSG was going to be difficult, this is going to be even harder.
Nicol’s co-pundit, Craig Burley, argued that United need to defend deep and prepare to have a tiny amount of possession tonight.
“There’s only so many things you can do,” he said.
“You can play a back four, a back five, you can leave one up or you can leave none up. There’s only so many things you can do on the pitch.
“They’re going to defend deep, they’re not going to have much of the ball, and I think they’re going to face a Barcelona team a bit better than they were at Old Trafford.
“Lionel Messi had a quiet game, although he created a bit for them, but his general play was quite quiet. I don’t expect that to happen tonight.”