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.