This match is highly unpredictable, which is why there has been much speculation about the possible outcome. The two teams are in their primes, playing in their respective domestic leagues; one team prevailing over the other at this point will be extremely difficult, and watching an unpredictable match will be extremely entertaining. This match is worth watching in the Europa League semi-finals or finals, and I always wish it came later in the Europa league.
Both teams face tough challenges in their two-leg meeting, of course, because each has quality players. I would never expect either one to be underestimated for whatever reason, but as far as what I've seen Barcelona football this season has been pretty good. This is signal that should probably be the reason why Manchester United can't let their guard down against Barcelona, so there's no reason to underestimate them.
The first leg will be played at Camp Nou, Barcelona are favorites at home but there is no certainty about the result. Marc-André ter Stegen who is in his best form is the reason why Barcelona's defense looks so strong in La Liga, and Barcelona's strength this season is almost evenly distributed in all positions including defender, midfielder and forward.