In the Europa League and Premier League, Ten Hag field completely different squads and have different match tactics. Manchester United's performance in Europa League matches is even weaker.
They don't prioritize Europa League even if they take that trophy, they will have a ticket to Champions League next season even they won't complete this season in top 4 of Premier League. However, both Manchester United and Ten Hag are realistic enough to know they are not competitive enough to win Europa League. Therefore they want to focus more on a long run in Premier League where they can fix mistakes but still be able to achieve good standing at the end.
Even if Ronaldo is with the squad, he is not playing well.
First of all, the club don't use a strongest squad for Europa League. Second, Ronaldo still has issue with his mind, confidence and body fitness too. His age has shown bigger negative impact on his performance in this season.
United face Omonia in their next match after a poor performance against Manchester City. Ten Hag should aim to improve his match strategy and strengthen his defense. If they don't improve their match strategy and defense, we won't get a better performance than Manchester United.
First they need to win it because without 3 points, there will be more drama as well as pressure on them.
Second they should take advantage of such small matches to train their players more, test tactics as well as improve quality of their squad in general.