In Barca-MANU, I feel pretty damn confident of a mismatch on paper. However, Barca do struggle in QF away legs. Nevertheless, I don't see much of a path to victory for MANU. Martial and Rashford have the speed to exploit the Pique-Roberto side of the D, but that assumes Roberto starts over the speedier Semedo (my choice for today). Lenglet and Alba are pretty much shut down on the other side and Rakitic/Sergio are brilliant at covering for Alba when he gets forward. Lukaku can be a load, but Pique and Lenglet are a formidable CB pair.
I don't see any combination of MF MANU could deploy that will trouble Barca's trio of Sergio-Rak-Arthur. MANU could and likely should try to jam up the midfield and play a 4-5-1 of sorts, but I don't know if they have the players or practice to pull it off. It would make sense for Lingaard or Rashford to harass Sergio all match and force Rakitic to be the motor. Then have Matic and/or Herrera to follow Arthur around a bit. That leaves Pogba matched up on Rakitic and I think that benefits MANU in that he can cover him and it will put him in good positions transitioning to attack. Both are huge workrate players.
At the back doesn't look much better for MANU. Ashley Young can handle Coutinho fine on his own, but I think Suarez eats any pair of United CBs alive. United doesn't have a single defender that can trouble Messi although Ashley's physical play is probably best, but he's on the wrong side. In any event, I think it's a case of pick your poison for United and I would cheat off of Coutinho and to a lesser extent Suarez and make those guys beat you. One thing to watch is for a late Malcom sub around 70'. Malcom has been playing slightly more lately and he has looked great. He has blazing speed and gives Barca another dimension that Coutinho doesn't offer.