The boss has been impressed with the way Ngog has led the Reds' attack in the absence of the injured Fernando Torres.
"David is doing well and he has deserved his starts because you can see that he is working really hard all the time in training," said the boss.
"When you do that, one thing is always true and that is you will get a chance. When [young players] are working hard, you always have to look to do that.
"If that is what happens, it is then up to the player to perform. So David has done well over the last month and has scored some important goals.
"We are much happier with him but we have some other players who are like that. Lucas is one who is showing great character and commitment every single day.
"The most important thing, though, is that they keep going. They have to keep working hard all the time, otherwise it will be difficult for them to get those chances."