The Chicago Bulls held a press conference on May 15th where Dr. Cole discussed the details of Derrick Rose's successful knee surgery.
You can see the full press conference for yourself on Bulls.com here.
Here's the highlights of the video, everything should be viewed as a paraphrase of what Dr. Cole said rather than direct quotes unless otherwise noted as my opinion.
What was torn
His ACL (duh)
Not his MCL
Two minor meniscus tears: a medial meniscus tear which was fixed and did not require any removal of meniscal tissue and a slight outer meniscus tear which was termed irrelevant.
Why was the surgery delayed
The initial goal with prehab (pre surgery rehabilitation) is to minimize swelling and get motion back. Derrick Rose had so much attention to his knee after injurying it that his prehab went exceptionally well and the swelling was low and motion was good.
This allowed him to get the surgery earlier than a normal person likely would have been able to do.
Cause of the injury
The believe is that it's most likely a completely random event. Most of the time that's what it is. It's possible that it's related to conditioning, but we'll never know for sure and the most likely cause is bad luck.
Dr. Cole specifically discussed the idea that other injuries were at the cause of this one and dismissed it, saying it doesn't necessarily work that way, and it's more complicated than that.
It will take eight months to a year before Derrick Rose see the court and longer before he's able to play at a pre-injury level. Connecting the neural pathways so the brain is in tune with surgically repaired knee takes the longest.
Some players can come back sooner than that time line and some later depending on their progress. The goal is to bring him back expeditiously, but safely. Derrick must go through various progressions in order to return to the court.
Age definitely works in his favor in terms of recovery.
Long term effects
It's impossible to predict for sure, but statistically he's most likely to be the same player that he was and make a full recovery and return to the same level of explosiveness.
His chance of re-injury in the knee is very low. It's actually more common for the sufferer of an ACL tear to tear the ACL in the other knee rather than to re-tear the ACL in the repaired knee.