Sorry to hear about your injured knee.
But unfortunately, this is what's likely to happen to you.
You call CMPB to inform them of your injured knee, and they
schedule another medical appointment for you. You go for it, and
the MO classify you as PES D. This means you will not enlist for
the moment. They will schedule another medical appointment for you
again to determine the outcome of your injury.
If you knee has healed, then you will be upgraded to PES A or B and
will receive your enlistment letter again. If they decide to
downgrade you to PES C, you will also receive your enlistment
letter. However, you will be doing a non-combat BMT and
subsequently non-combat posting.
You will not get exempted from the army. I suggest you get a letter
from your hospital or specialist clinic about your injury and call
CMPB to inform them.
To be exempted from NS means you need a PES F grading. PES F
usually involves people who are mentally unsound, serious physical
disability (one leg, one hand). I'm not a MO so i can't say for
sure. But for your case, i am very sure you will not get
exempted.