It is only a season-ender because he opted to get it repaired.
Think of the meniscus as a little rubber endpiece under a knee.
His is basically flapping. He could have had it shaved off or repaired.
Had he shaved it he would have lost the shock absorber effect but had been back soon.
Repairing it means staples which apparently knocks him out for the whole season.
Not every torn meniscus can be repaired and the fact that his is means that it was a substantial tear.
This couldn't have been an easy decision. In the long run it is better to have more meniscus, but he loses yet another year in the prime of his career which has to hurt his earnings potential for the next contract. Of course it kills the bulls.