It had been a while since Manchester United and Barcelona had match, I don't think it is ideal to use their H2H to measure their capacity because the last time the two teams played, Messi was still in Barcelona and if you look down from that moment to now, lots of changes have happened since then.
Sometimes history in football repeats itself because irrespective of the players and the coaches presently in the team, the rivalry remains the same; Taking Barcelona and Manchester United case into consideration, all the teams are after the record they have in the past therefore both teams will like to win the match and bring victory to their team because they are all hoping to be Europa league champions this season.
Barcelona is currently managed by Xavi who knows the ways of Barcelona and has improved their form and Erik has done his own part in fabricating Manchester United in its own capacity, so the outcome of this match is really a tough one to predict but recent statistical performance may favor Barcelona little.
Statistically, both teams are playing very well in their respective leagues; Barcelona has won all their last five matches while Manchester United won four and lose one to their league leaders Arsenal which is normal; You are right that the match result will be very difficult to predict but there is a slight hope for Barcelona especially as they will be the one to start hosting Manchester United.