I don't think so either because apart from Arsenal and Barcelona there are also other top opponents in the Europa league this season who are capable of winning, teams like Juventus and even Manchester United will definitely be tough contenders for the trophy but as an Arsenal fan I will be supporting my team to win it in the end which I guess should be understandable lol.
You cannot be counting Barcelona as favourites to win the Europa League when they have not overcome Manchester United that is a big challenge for them. If you want to count favourites, it will.be fine you mention both of them because anyone that won between Manchester United and Barcelona may likely get to the finals of the Europa League for this season. It is unfortunate that the two big teams met early
It is however obvious that they are favourites against United, and that also means if they can beat United, there are not that many teams left which Barcelona would not be high favourites against. This still doesn't mean that they will win it, Frankfurt wasn't favourites against Barcelona last year and they still managed to beat them, which means Barcelona could win against United and then lose to a much lower level team, we do not know.
This is just a general talk about who has the highest chance to win, but even on a 30% chance to win, which would be higher than any other team, that still leaves a combined 70% for all the other teams and that is the thing we should remember.