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.
Do not underestimate Arsenal and Roma, they're a strong clubs without a crown. Juventus is a mediocre club, their performance isn't stable and they got beaten for 5 goals against Napoli, it's really a worst result for Juventus. Current Manchester United performance is really strong and they've beat Barcelona, but the goal difference is only a goal, Manchester United doesn't really humiliating Barcelona. So I believe Arsenal, Roma and Manchester United are equal clubs.