No-one thinks Sancho has been playing well but there is reason to believe that there are reasons outside of his control as to why he's not playing well (Although there are reasons why he himself if not playing well that are within his control):
1. Poor form: This isn't really that much of a concern, players go through hot spells and cold spells, no reason to worry much on this front
2. Lack of athleticism: He's a really good technical dribbler but he completely lacks any burst. If you don't have burst, you're not going to be able to beat your man. I also think that when you're playing out on the wing, you're a lot more isolated so you kinda have to be able to win 1v1s. He was able to get away with his lack of explosiveness in the Bundesliga but not in the PL. I actually think he'd be better as a #10 than as a winger because he's technically gifted, has great vision, and is less boom-and-bust than Bruno (And also a better carrier of the ball from deep)
3. Lack of intensity: This is the biggest issue. As some Americans like to say, he just hasn't got that dog in him. I'm not saying he can't improve on this but it's impossible to be a top-class player without playing with high intensity. Funnily enough, he can do it; when he came on versus Omonia at OT, he brought real energy to the game. But he needs to show this game in, game out in order for him to be a successful Utd player (Or a successful player anywhere for the matter)
4. Bad fit with personnel: Because of his lack of athleticism, he's reliant on link-up play to create. If you look at him when he was at Dortmund, I'd say that most of his best moments came less from beating a man 1v1 but more from clever link-up play with other players. If the LB doesn't make a run for him, there's one link-up play option gone. Bruno prefers to go for the killer pass rather than link-up play and he'd rather play with a runner on the left rather than Sancho. Rashford and Ronaldo don't help either as they don't like linking up that much and aren't that good at it; Martial is a big loss in that regard. If we improve our personnel going forward with more players who like to link-up, then Sancho will directly benefit
5. Bad fit with (current style) of play: Don't get me wrong, we've seen some real improvements in how we play and out build-up play has come on leaps and bounds. But if we take the Newcastle, Spurs, and Chelsea games as a whole, how many good chances did we really create? Even in the Spurs game, a lot of it was shots from outside of the box. Our game in the final third needs some refinement; as this part of our game improves, so will Jadon.
I think when you put it altogether, it's not all doom and gloom. Sancho can be a very good player for us, maybe not a properly elite producer but a very good player all the same. Maybe that requires a position changes, maybe it requires Sancho to improve mentally, maybe it requires the rest of the starting XI to improve around Jadon. But there's no need for knee-jerk reactions; this process was always going to take time!