Yeah, he's been out for ages. Kinda sucks, because his (and Hayward's) playmaking has been missed at times.
Annoying that he seems to get weird flukey injuries somewhat often
That's because he's very intense.
He plays so hard, he's always diving on the floor and taking charges.
The human body just can't handle that kind of abuse.
Larry Bird was able to for a near-decade 
That was a different era.
Players right now are stronger, faster, and heavier.
Larry Bird was 6-9 and only weighed 220 lbs.
Jaylen Brown is only 6-6 but he weighs 222 lbs.