Torn Meniscus
It turns out it wasn’t something that rest and physical therapy could ultimately help.
I had torn the medial part of my left meniscus. The doctor said it wasn’t from a particular moment per-se, but rather just wear & tear given my activity level and age.
It is the most common form of knee injury for athletes.
So after three months of PT and two months of rest, an X-ray, and an MRI, the fix took less than two hours - and supposedly the entire procedure was only 20 minutes for the doctor.