Whenever you sign an elite player, like Sanchez, Mkhitaryan or Di Maria, there is the danger that the previous system they played in contributed significantly to their impressive performances. Sometimes the player will adapt to United's style of play, perhaps our players will make subtle changes to accommodate him.
Whenever you sign a player aged 29, especially a player that relies on his pace to get away from opponents, there is the risk that he is past his peak and his performances will decline rapidly when he is into his 30s. If you are lucky he will still perform at a high standard aged 30, possibly even 31. Sanchez had played a huge amount of football before he arrived at United, yet another warning sign we ignored.
It really shouldn't be a surprise that Sanchez has failed so spectacularly here. 50% of transfers aren't a success.
A signing like Lindelof or Dalot can have a difficult first season without a huge amount of drama. With Sanchez he had to start fast, demonstrate his value early on. Be a massive success before the inevitable physical decline impacted his performance level.
In his case, both managers appeared to lose faith in him. Mourinho went from starting him almost every game he was available last season, to roughly half this season. Under Solskjaer Sanchez started one third of matches he was fit. The best paid player at the club is in reality a squad player, a sub when everyone is fit, starts when there are a few injuries; an embarrassing situation for all concerned.
I'm against buying any out-field player aged 28 or older, simply because successes are rare, while failures are common. I'm also not enthusiastic about paying incoming transfers higher salaries than United's best player; we've seen a lot of that in recent years.