Apart from Arsenal, Barcelona, and Juventus, I don't see any other strong contender that is capable of stopping Manchester united in the Europa league. Eric Ten Hag has done a great job so far, and I believe he is setting every system to go for the Europa league title. Manchester United is so hungry to get a title for some seasons now, it will be so good to see them register another Europa league title after the last one in 2017 in Sweden.
Feyenoord and Real Sociedad are strong clubs too and both of them can compete against top clubs, you shouldn't really confident to see Manchester United will become a champion because their first step is need to beat Barcelona first, Barcelona is strong and have beat Real Madrid in this season. Juventus actually a bad club in this season, the management have an internal problem and it will cause the players get pressure too.
The Real Sociedad team is perhaps better known than Feyenoord. But I consider Feyenoord as the favorite in this match. The Feyenoord team has been consistently performing well in the Eredivisie league. They are at the top of the table. And lost just one match. My guess is Feyenoord will win against Sociedad. On the other hand, the match between Barcelona and Manchester United is unpredictable. Barcelona is in very good form now, and they are capable of beating any team. On the other hand, United's squad still has some problems. Their performance is not regular.