I've always thought this, it's just that Nogueira seemingly doesn't get any better even with the undeserved increased playing time. Poeltl comes in and you can notice how much better he is at getting himself into good positions all the time, whether it is on the glass or making himself open to save one of the guards who are driving and run out of room.
It obviously has a lot to do with him being a noodle, but Nogueira just gets destroyed by opposing bigmen. He rarely ever gets a hard fought defensive rebound, if it doesn't land in his hands most likely he allows the offensive rebound or tries to tap it away to a teammate - he has an insane wingspan, he should really try catching the f***ing basketball! I'll give him that he's good at using his length sometimes to pick up blocks when he has space - on the contrary if he's close to a defender it's almost always a foul which he never agrees with - and he can be a helpful PnR partner for Lowry who just lobs the ball close the rim while drawing 3 defenders and it's as easy as an NBA alley-oop gets.