I agree with you, United needs to reserve their strength for the match against Lyon, because that's the most important match amongst those three matches. United should take the Europa cup more seriously because that's the only hope for them to win a trophy in their current predicament, and also qualify for the Champions League next season.
I am quite sure that Manchester United will prepare themselves very well for the rest of the season in the Premier League along with their preparation when they meet Lyon because if we look at the chances in terms of trophies this season for Manchester United, it is only in the Europa League so their efforts will definitely be much more serious in the Europa League even though in the Premier League Manchester United will also try to win as much as possible in order to maintain their position not much worse than it is now.
I think Ruben Amorim is a realistic person who can assess a situation, so of course he will prioritize the Europa League above all else. Manchester United has no hope left in the Premier League, and I think finishing in 8th or 15th place is the same, so why bother if it can't turn the situation around? The most important thing is that they don't just end up in the relegation zone. If we look at the current standings (20 points difference with 18th place), 2 wins and 1 draw should secure Manchester United.