His return to the line-up is far from obvious to me - maybe Ten Hag will decide to punish him further and "show who's in charge here." Or Ronaldo himself will not want to play in the Europa League, I think he is ambitious enough to ignore United already and wait for the World Cup and the winter transfer window.
In addition, if Ten Hag decides that United should take first place, then he can play decisive games with the main squad, where Ronaldo is not currently included. And by the way, judging by United's recent games, I would say that Barcelona should try to avoid meeting it and not vice versa.
They are usually better than the teams they are playing, which means that they can afford to play them on Europa right now, in the league they are playing against very good teams, and it's harder to bring in him, I would say instead of all this trouble, if they kept putting him in at 70th minute or so for like 10 games at the start of the season, that would have given him enough match shape that he would be able to play all game, or you could have put him at start, and bench him at 65,70th minutes so he could rest.
That is a possible thing and would have worked for them. Instead, they are doing it "bench him on EPL, play him on Europa" style, which is weird.
In general, the problem of Ronaldo at United is in the past for me. I hope he leaves in the winter and ends up in the right team for him, I think he will easily lower his salary expectations in order to continue playing at a high level.
As for United in the Europa League, Ten Hag could "accidentally" wash this tournament away to fully concentrate on the Premier League, so it's hard to predict.