My point is not a judgement of Nelson but a scrutiny of the statement that Guendouzi and Nelson are 2 of the best teenage prospects around.
The "joke" you do not get, is similarly aged players ahead of them, when it comes to cementing their places in their respective teams.
Using your logic, i could say Nelson cannot get into a Europa team and that's why he is on loan to a lower level team.
This is reflected by Hoffenheim being the 3rd lowest ranked Champions League side.
Viktoria Plzeň (CZE)
UEFA coefficient ranking (end of 2017/1: 45
Club Brugge (BEL)
UEFA coefficient ranking (end of 2017/1: 50
Galatasaray (TUR)
UEFA coefficient ranking (end of 2017/1: 51
Young Boys (SUI)
group stage debut
UEFA coefficient ranking (end of 2017/1: 74
Internazionale Milano (ITA)
UEFA coefficient ranking (end of 2017/1: 83
Hoffenheim (GER)
How they qualified: 3rd in Bundesliga
group stage debutants
UEFA coefficient ranking (end of 2017/1: 99
Crvena zvezda (SRB)
UEFA coefficient ranking (end of 2017/1: 119
AEK Athens (GRE)
UEFA coefficient ranking (end of 2017/1: 123
https://www.uefa.com/uefachampionsleague/news/newsid=2563715.html