Arsenal, Roma and Juventus are not better than Manchester United. If Manchester United can eliminate Barcelona their most deadly rivals, they can do that to any team. It is that simple. The winning mentality of Manchester United is so high for now. The new coach of the team carries pressure and the players are more United than before.
If we are talking about the Europa League, then Roma and Juventus >> than United and Arsenal. The circumstances are such that Juventus don't give a damn about Serie A at all, Roma seems to have a secure place in the top 4 and they just need to keep it. For these two clubs, winning the Europa League would be a welcome accomplishment, given that they are not in contention for the domestic title. United and Arenal are fighting for victory in a more prestigious tournament (Premier League) and their main forces will be sent there.