Nobody knows how Eric Ten Hag was able to convince Sancho before he decided to stay in the club and I believe that there must be a reason for Sancho's decision. Don't forget that this is a player that was not in good behavior to his coach but suddenly he decided to become submissive to his Boss, so I believe that Sancho will not be on the bench and don't be surprised to see him among the first eleven because his performance in Dortmund was good.
What you say seems reasonable enough to not be surprised when everyone sees a player like Sancho at Manchester United as a main player again. Because what Sancho showed last season at Dortmund in every important match was enough to provide proof that he is the best player and he even got to experience what it was like to play in the UCL final last season even though he was ultimately defeated by Real Madrid. However, what is being taken into consideration is not that, but rather the fact that Sancho still has good enough abilities in any squad so that he is still worthy enough to be the main player in the team rather than a reserve player.
Although Sancho returned to Manchester United, but there is no guarantee that Sancho will have the same good performance as Sancho playing in Dortmund. Because after all, previously Sancho also played well in Dortmund and then Manchester United bought Sancho but what happened was, Sancho's performance was not as expected so Manchester United loaned Sancho to Dortmund again.
In such a way, I personally remain convinced that there is no guarantee for Sancho to be able to provide good results at Manchester United. What's more, there are also rumors about Juventus interest in buying Sancho and not only that, because there are also rumors that Juventus are ready to exchange Chiesa to bring Sancho from Manchester United to Juventus. With rumors like this, it seems like any decision could happen, because Juventus will also play in the Champions League next season, and Manchester United only play in the Europa League next season.