Did you watch the Spanish Super Cup final which Barcelona just won. Barcelona played like crazy to win trophies, the struggles of Barcelona players were not in vain, they managed to beat Real Madrid with a landslide score.
Let's compare the Super Cup competition with the Europa League, it's impossible that Barcelona don't care about the Europa League trophy. The Europa League competition is more prestigious, of course Xavi will prioritize the Europa League this season, especially since Barcelona failed in the final last season.
I'm sure Xavi's priority right now will be La Liga, especially since after beating Real Madrid they will have enough self-confidence to keep their lead. Barcelona won't mind winning the EL as well, but the first game will be against the MU, who are also in great shape right now and they beat City this weekend. We can say that both clubs are close to their optimal form, it should be a very interesting confrontation.
I also have no doubt about this Xavi is having first priority in La Liga because if they are able to win this definitely it would be good for them as in next season they can do better with confidence and having something to cheer as well.
But, if they have good performance and confidence then surely they can beat Manchester United in Europa Cup as well because right now they have just three points lead and Real Madrid is also trying their best for narrowing this all because they are also defending two titles with few injuries issue, but things can change any time for them as well they have good players, but their moral is currently down with they need to wake up nothing specially which is currently missing from their side.