We're in the middle of the season, a period clubs in Europe will do everything to maximize the potentials of their players and at such, I don't expect Manchester United to let an important player like Alejandro Garnacho to leave the club at this time and join Napoli. Just like you said, they need the Argentine is a big asset for the Manchester club as they fight to win the UEFA Europa League competition this season.
Marcus Rashford is the player that the manager have deemed surplus to requirements at the club and if there's any player that's more likely to leave, I think it'll be him. Manchester United are one of the favorites to win the UEL trophy this season and if Ruben Amorim can continue his impressive run in the competition, I think they might end up with the trophy
I agree with you, it is impossible for Manchester United to let go of Garnacho in the midst of an ongoing competition. Because if Manchester United did that, it would be the same as making their team weak. Therefore, I am also sure that Garnacho will definitely be retained by Manchester United at least until this season's competition is over. Apart from that, if for example Garnacho's performance is very good with Manchester United, the chances of Garnacho being released after this season are finished are certainly quite small. Because it cannot be denied that currently Manchester United is building its performance to be better. So a great player like Garnacho cannot possibly be sold to any team.
But it's a different story with Rasford. Currently he has a bad relationship with Amorim. So even though Rasford's performance in my opinion is quite good at Manchester United. But if he has a problem with Amorim, I don't think it will have a good impact on Manchester United. For this reason, if Rasford is sold by Manchester United, this might be the right choice. Because as far as I know, currently Rasford is rarely given many playing minutes by Amorim. So even with this, it can be indicated that Amorim really doesn't want to see Rasford at Manchester United. That's why Manchester United deserves to sell Rasford more than Garnacho.
And it's true, if for example Manchester United can maintain its great performance in the European league, the chances of becoming champions could be said to be quite large. But even so, Manchester United should not underestimate the teams playing in this competition. Because in my opinion the teams that entered the round of 16 are definitely all quite great teams.