THe thing I really don't get is, Sanchez was constantly crap for Arsenal. He got goals but, hell even before we signed him I said ultimately he plays a bit like Walcott. he wastes time on the ball, he doesn't achieve much out of the box, but you play him into the box to shoot on his first or second touch or the ball drops to him in/around the box where he can shoot first time and he's a completely different class to Walcott. Unlike Walcott he does love to dribble but most of the time he loses the ball doing it (same as Walcott there just that WAlcott's also a giant vagina and passes backwards/sideways rather than actually try to beat players 9/10 times). He's exceptionally wasteful, selfish, often slows down plays with his insistence on dribbling and he throws himself to the ground A LOT. I said all this before he joined Arsenal in describing how he plays and I said it throughout his time with us.
He can still be effective because given those chances without much to think about he's 100 times better than Walcott at finishing but he brought more to the team in goal scoring and assists (on a similar thinking, give him the ball to cross or pass without thinking in a position where that can turn into an assist and he's good, dribble 20 yards then pass.... not so much).
At Arsenal you had a slow fairly useless striker who was used as a backboard for passes, this worked for Sanchez, give Giroud the ball on the edge of the box and pass sideways for Sanchez to run onto and hit, or have Sanchez come in on end of a cross like Freddie or Pires. With Utd you have more players who like to keep the ball, a striker who can run with the ball and is dangerous from more positions and other players who are effective and Sanchez is being asked to play more of a support role for the striker rather than at Arsenal being the guy being supported by the striker and other attackers.
Sanchez was always massively flattered by his goal scoring at Arsenal, his general play was monumentally over rated and him getting something stupid close to 400k a week is beyond crazy.
Utd are stuck with him, no one will take him off Utd's hands with that kind of wage. There is a reason Guardiola saw no point in offering him silly money or a huge transfer fee.