Erik ten Hag's side hold a strong position heading into the second leg at Old Trafford next week, with Marcus Rashford in fine form once more, as he netted his 22nd goal of the season during the encounter.
Former Chelsea defender Marcos Alonso opened the scoring in Catalonia, before a quickfire response from the Reds saw Ten Hag's side turn the scoreline around in just seven second-half minutes, with a Jules Kounde own-goal following Rashford's equaliser.
Raphinha answered back for the hosts, who unleashed a late onslaught, but United's players dug deep late on, with it all to do again back in M16 next week.