Lionel Messi has scored 43 goals in 40 matches as the Catalans have surged to the last eight of the Champions League as well as the final of the Copa del Rey.
The 31-year-old Argentine was on target on Saturday night at the Camp Nou in the 2-0 win over Atletico Madrid that took them 11 points clear of their opponents at the top of La Liga.
"Messi is a fantastic player and will go down in history as one of the best individual players ever," said Manchester United manager Ole Gunnar Solskjaer.
"He will be difficult to stop but it's not like it's Messi against Manchester United.
There is Luis Suarez, Philippe Coutinho, Ivan Rakitic, Arturo Vidal. There are so many good players. We can't just focus on one player. We know we have to play against 11 of them."
United's never-say-die spirit was on supernatural show during the second leg of the last 16 match against Paris Saint-Germain at the Parc des Princes.
Trailing 2-0 from the first leg, United refused to cede against a technically superior PSG and the home faithful were stunned into silence when Marcus Rashford scored the vital third goal in second-half stoppage time to seal a 3-1 triumph for the underdogs.
United will again carry that tag in the first leg of the quarter-final. But it's a deceptive label, said Barcelona defender and former United player Gerard Pique.
"The game against PSG is a great example. PSG were better. But you cannot let United off the hook because they have a great team and a great history as they showed in Paris.
"We have learned that things happen, even if it seemed unimaginable that they will. We've learned that you cannot let an opponent back into a tie if they are not killed off. That will help us for facing a team like United. If at any point we can kill off the tie, we have to do it."
United have lost three of their four games since victory in the French capital on 6 March to crash out of the FA Cup and lose ground in the race for a finish in the four Premier League places that lead to next season's Champions League.
Solskjaer says his players believe another coup is possible in the Champions League. "With the quality of the Barcelona team, we know we need to step up our game," said the 46-year-old Norwegian.
"We know we need to perform to our best levels, which we didn't against PSG at home in the first leg. The outcome of the second leg against PSG has given the players and supporters belief that the same is possible against Barcelona."