Romelu Lukaku has called on United to play with "more heart and b***ocks" in the Nou Camp as they seek to overturn their Champions League deficit against Barcelona.
United travel to Catalonia next week with their hopes of reaching a first semi-final since 2011 hanging by a thread after Barcelona won 1-0 at Old Trafford on Wednesday.
An early Luke Shaw own goal has handed the La Liga leaders the advantage in the tie, but United can take heart from the fact they've already won away at Juventus and Paris Saint-Germain this season.
They also matched Barcelona for much of the first leg at Old Trafford and Lukaku believes if they take their chances they could still progress to the last four.
But asked what the key to success in the Nou Camp was going to be, the striker said: "Taking our chances and not conceding a goal, and play with more heart and b***ocks."
United overturned a 2-0 first leg defeat against PSG, winning 3-1 in the Parc des Princes and progressing on away goals courtesy of Marcus Rashford's dramatic injury-time penalty.
But given Barcelona haven't lost a home game in the Champions League in six years turning this tie around would rank as an even bigger achievement, and will require an even better performance than United produced in the French capital.
"Barcelona is a different team, every game is a different game, we just have to improve and do better than we did in Paris," added Lukaku.
Ole Gunnar Solskjaer might be able to call on Alexis Sanchez to face his former side next week, with the Chilean back in training after a knee injury.
While Sanchez has continued to disappoint under Solskjaer, Lukaku has backed the 30-year-old to come good and he believes he could yet reemerge as one of the Premier League's best players.
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"He is a very positive person and he has a lot of respect in the club and for me," the Belgian said.
"The most important thing is that he is the happiest he can be and that us as players must give him more confidence, because with the qualities that he has he can be one of the best players in the league.
"The way he trains every day, he shows us his quality and for me I hope that he can help us finish in the top four and go on in the Champions League."
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