You know it's really hard for players that are at the peak of their career to join struggling teams? Sesko had a good season with Leipzig to to be frank this is the first time I'm hearing more about him and he has the chance to go to a team that his going to play an European football were his likely going to build more on his career for a team that will only be playing domestic games. It will really take lots of convincing for a start especially for a player that doesn't even have the team at heart. Yes, Manchester United is a big club but for now they are just a shadow of their former self.
Manchester United don't need to convince Šeško to join them this season because Šeško has made it known to Leipzig and Newcastle United that he wishes to play for Manchester United, just the way Gyokeres made it known to Sporting CP and Manchester United that he wishes to play for Arsenal. The ball is now in Manchester United's court, and since Šeško wants to join Manchester United, the only thing that will stop Manchester United from signing Šeško from Leipzig is if Manchester United fails to meet Šeško's transfer fee.
If he plays well in Manchester United and strengthens them, and Manchester United is also not a good team, which is why Manchester United should include him in their team. So this is a good thing, this will also give Manchester United a good effort, due to which they will become stronger to some extent and this is also a good decision for this player. Now Manchester United, I don't think should let this player go but in return, he is getting a good and excellent player due to which Manchester United will let him go and let him join the Arsenal team. This is also a risk for Manchester United because I think they are not ready to include him. I already felt that Manchester United didn't want to include him because they didn't want him to play in their team or it could be that Manchester United didn't have the money for him.