First match is going to play after two weeks at Red Bull ARENA even AS Roma is superior in quality and performance, but surely this is not going to be easy game for them because Salzburg is having good time in Champions League, and they play draw against Chelsea and won against the Zagreb in their Champions League group matches which give them chance to have spot into UEFA Europa Cup
Salzburg is like a good opponent for AS Roma. They are not too strong and Roma are able to beat Salzburg but if Salzburg could have a draw against Chelsea that is a bigger club than Roma, they can do the same or better against AS Roma.
Roma probably sacrificed Coppa Italia to focus more on Europa League and Serie A so they will be able to use best players they have to play against Salzburg. To beat Salzburg, they will need full fitness from Dybala and Pellegrini who both got problems recent weeks with their fitness and were not actually ready for full matches.
What you were saying was true. MU has very bad record against Barcelona. MU was so difficult to beat Barcelona.
In the past twenty years, Barcelona are stronger than Manchester United so it makes sense to see their face to face result is better for Barcelona. In this season, they look to have similar quality but Barcelona are still a little bit better than Manchester United. Again maybe Barcelona will beat Manchester United and become a winner after two matches.