The match between Barcelona and Man Utd looks like a 50/50 to me. United will have a point to prove after letting go of Ronaldo and Barcelona are just having issues at the big stages. I'll put my money on Barcelona to win anyway.
Man united need to work on their midfield and attack if they're going to compete for any trophy in the future. They just do not have the team to be among the top teams.
I think Barcelona have more chances of winning Manchester united. Barcelona has Lewandowski who can score some number of goals, this is an advantage for Barcelona. The game will be interesting, this are the big clubs in the Europa league.