I don't think Manchester United will be treating the Europa League as their second priority as you just said. Manchester United has stepped up their form a bit and is putting in more consistency in its winning, but I will not be fast to say that they will be able to overthrow Manchester city not to talk more of Arsenal who seems unyielding and indomitable this season. For that, man united will be more focused on the Europa League than The EPL. The only problem is that they will still have to defeat Arsenal before they could secure the Europa title. For now, I see Arsenal as man united greatest Rival.
That is the toughest problem for Manchester United this season because in the Europa League there is also Arsenal and in the Premier League Arsenal are also still leading the top of the standings which will also be very difficult for Manchester United to unseat Arsenal. And actually this kind of thing will also be very interesting to see in the Europa League because both of these teams will both focus more on two different competitions this season.