Both youngsters have played like Premier League veterans so far this season
But Di Matteo warns: "The players that have come in have settled in quickly and that has been a big bonus.
"Oscar is talented but I don't forget he is still only 21, and so is Hazard, and we don't want to give them all the responsibility in the world. We cannot forget that even though they are important players for us and for their national teams too.
"They have come to a new country and a new league, and they are learning a new language and getting used to the high tempo English football.
"Oscar is playing all the games for his national team too and doing a lot of travelling, and you need to factor all those things in.
"We have had a good start but we need to show consistency over a long period of time. We have a very busy schedule coming up and that will be a test for us and the depth of our squad."