training a solid U21/NT performer is a hard job. Although you can arrange training in both (team + NT/U21) interest, it is something you have to struggle with. Your trainee becomes the priority, all other trainees should adapt, you will spend weeks and weeks in monorole.
All that is not necessarily bad, anyway. My team is where it is because of my former U21: I caught the opportunity and built around. And, despite training a potential future U21, I am fully aware of the sacrifices that it can bring. Not all managers can train them well at any time.