Barcelona's performance is not stable yet. There are many star players in the team. However, Xavi is still struggling to stabilize the team. Barcelona is going to face Manchester United in the first match of the Europa League. Manchester United team is still not stable. Their performance is weaker than Barcelona's. And so I think Barcelona will probably be able to win against Manchester United. However, Xavi's main goal now is the La Liga title. However, Xavi will surely form his best squad in the Europa League matches.
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.