Emery was speaking after their preseason defeat to Club America in the US.
On fullback Nedeljkovic, Emery said: "He is one of the players I am speaking about - his adaptation is going well, but he needs time to understand tactically of us. Of course, he is only 18 years old, which is very young, and I am very happy because he is a very good person, good professional.
"In training and in matches, he is being very alert, how he can improve himself. This is the way. He needs time, but I am very happy with him."
On midfielder Onana, he also stated: "He is a player who is demanding himself, he is always trying to understand quickly what we are doing in every session we are doing and every meeting. He is only 22 years old, of course his potential is very interesting for us.
"This is our challenge and his challenge, but his progress with us is going well. We want to add his quality and potential trying to progressively be stronger in our structure tactically with his skills and characteristics at the end."