Erik Ten Hag did not handle the issue of Jadon Sancho even if it was the player that made the biggest mistake. There are some comments that a coach shouldn't make against his players. Such comments can reduce the value of the player. He wouldn't have publicized that the reason for dropping the player in the game was because of lack of fitness. And Sancho wouldn't have also countered his coach publicly. This issue would have been handled internally.
Sancho was asked to apologise but he refused. His refusal propelled Erik to ban him from the team. He doesn't train, eat it even stay in the dressing room with the senior team. But the English player has made his decision to move to his former club in January. Manchester United might ask for as much as £40m from Borussia Dortmund.
I don't know what fatal mistake made by Jadon Sancho and he withdrew from Manchester United main squad, currently ten Hag have warning Sancho ask apology and will get second chance return to the main squad both side on highest ego each other and difficult for one of them ask apology. For winter transfer window opening left two months later and keep on highest ego make Jadon Sancho have absent for two months later with Manchester United, I don't think good career for him although he want leaves Manchester United on next window transfer opening because he was absent for longer time with Manchester United.
Not difficult to find new team who interested for signing Jadon Sancho based on price bid offer from United, I think move to team have guarantee get more minutes playing is better for Jadon Sancho although not priority with top Premier League teams.