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.
You can search and read more about Sancho and Ten Hag's story in Manchester United. In short words, Sancho was out of the team for weeks because Ten Hag thought he was not ready and when they asked Ten Hag about Sancho he said this payer was not ready but just after that Sancho said he was completely ready for games and this is the reason for Ten Hag to keep him out of the team. Regardless of what happened between these two, I'm sure knows he really needs to have this palyer in the next games to improve his team.