Arsenal are focused on the Premier League, I'm sure they don't think much (if not at all) about the Europa League. As for Barcelona, even if it is a favorite, the bookmakers quotes are simply ridiculous:
UEFA Europa League 2022/23 - FC Barcelona reach Round of 16?
Yes - 1.57
No - 2.25
Being able to beat underdogs with a great percentage of efficiency is certainly good, but in big games Barcelona are weak this season.
Both of Man United and Barcelona are doing wonderful
But still defending very well is Barcelona
Man United will find it hard to break through
Playing well in La Liga (losing head-to-head against Real Madrid) and playing well in the Premier League are two different things, don't you think? In the Champions League with comparable opponents, Barcelona have been extremely bad, so I don't see any reason to give them such a big advantage against United.