The former Vitesse Arnhem midfielder arrived last week.
Mourinho said: “Marco is one of the best kids in European football right now. We had also been following him for a long time and he has great potential.
“We don’t have to push him too much — try and help him settle down and find out what it’s like at a club of our dimensions.
“Chelsea are investing for the long-term. We want to do important things now but we also look to the future and are investing in the future.”