Depends on several things, if he has strained or torn it, and also if he has done it to his posterior cruciate ligament or his anterior cruciate ligament.
If he has strained either of these, he could be missing for anywhere between 2 to 6 weeks. If he has torn/damaged it, your looking months... 4 to 12 months...
Dare i say it, a bad tear has been known to end a footballers career.