Just to put into perspective. We will probably need 100 odd goals or more in all competitions for a succesful season (4th place and competing for a cup or two). You have to be able to realistically work out where those goals are coming from.
Even if we push the "realistically" part, it's a struggle. You can say 20 goals each for Rashford and Martial if they both have very good seasons and avoid significant injury layoffs. Maybe 10 for Pogba if we start using him further forward at some point. Lingard, James and Mata might get 20 between them at a push. That's 70. You still need another 30 odd from somewhere else...and I think more likely you're looking at only one of Rashford or Martial getting 20ish, depending on which one plays as the striker more. Lingard and Mata could just as easily get about 4 goals between them as they could 10-20. So more likely you are looking at relying on between 30-50 goals from the rest of the team.
Even with Sanchez, this is a big ask, but at least with him, you have a player who has the knowhow and potential to get a chunk of those goals, particularly when you consider the likely standard of opposition in the Europa League. Take him out, and the bulk of this responsibility then rests on Greenwood, Gomez, Chong, etc.
Again for perspective, if Greenwood were to score in the Premier League, he'd be one of the youngest players ever to score a Premier League goal, full stop. Never mind score them regularly. The list of scorers younger than or similar age to him mainly consists of players who scored a freak one off goal at a young age (Macheda, Milner, etc.). The list of players his age who were scoring goals with any kind of regularity at all is as follows: Wayne Rooney...and that's it. Even Owen was 18 before he burst onto the scene.
So without Sanchez you go from asking for something that's unlikely to asking for what is essentially the impossible, as well as heaping a load of pressure to produce the impossible on a bunch of unproven teenagers who barely have 90 minutes of first team experience between them.
For someone to sit there and consider this and then conclude it's a good idea to pay another team to use Sanchez instead of us is frankly just plain self destructive. I mean unless he's walking into training and actively making everyone else worse, what possible sense does this make?