The Dutch midfielder underwent surgery yesterday.
Mourinho said: "The surgery went well, the doctor is happy with the way the surgery was made. Now he has to wait for a long time. It's very sad for a kid who was in an important season for his development but that's football, sometimes it happens. We lose a player for almost the whole season, I think five months, six months, it's a long time. To be back when the season is hot and the clubs are playing crucial matches, to be back directly for that, we don't know.
"The important thing now is only the kid. He is in good hands. We have very good physios here with experience of cruciate ligaments. The kid will be like new, for sure."
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