I watch Barca's evening games almost regularly because of Messi and they really aren't that special if you put him aside. obviously, that may sound stupid because we know he WILL play and we know how good he is, but there are quite a few things they struggle with and fortunately, we have players capable of hurting them.
first of all, they aren't that good defensively compared to last year and even then, while having Umtiti fit and in form, they conceded 3 against Roma away and 1 on Camp Nou and also against Chelsea away. even the teams you would consider average manage to put 2 or 3 behind them these days. it doesn't matter if it's Atletico they're playing against, or Valencia, or Sevilla, or Betis etc. they simply concede every time so there's really no excuse if we can't do the same as those teams.
obviously they're on another level technically but seem rather slow in midfield and there's basically no physical player there - at least not the one that could handle Pogba if he 'turns up'. even Lyon had no trouble getting to their half untill Messi killed the tie. if there's anything current United side don't lack, it's pace. when I see what some other, less known players are doing to them and then look at Pogba, Martial, Rashford, Lukaku... I can't help but feel confident any of those players can and will do some damage to them. they aren't really capable of hiding the ball completely from opponent like they used to be so I'm sure we'll get our chances.
another thing, Dembele is injured and they play Atletico today, so it's yet another game they can't afford to rest Suarez, Messi and others. as it's the only big game left for Atletico to play, they surely won't make it easy for them, especially when their win would mean it's only 5 points again between them.
of course, with all that in mind they are still better side, but the pessimism here is as if they're coming to play Getafe. we won't contain them and they'll score, but they also won't contain us, that I am certain of. it will most likely be high scoring game, with both team scoring at least 2 in my opinion.