The back is a major component you left out of your initial post. That is why I say to find someone skilled to evaluate you.
This is a dermatome chart. You did not specify what portion of you back you had surgery on. You can see from this chart that your L3, L4 and L5 innervate the region of your knee. It is possible that the two injuries are related. The knee weakness could have been due to low innervation of the region, resulting in instability and injury of the knee, which destabilized the back further. Lots of people live with herniations and have no idea until they become even more stable. I'm speculating, so is everyone else, which is why I'm suggesting you see a specialist that can evaluate you. Even just once to know where your issues are is enough.
Just wanted to add to my previous post now that I'm not on my phone.
I doubt the two injuries are related, the pain in my right knee started in my early teens and didn't get addressed by surgery with the full-on meniscus tear until I was 19. It was most likely caused by overuse when I was in year-round swimming since elementary and then varsity swimming in high school.
Back surgery probably related to poor posture, the scoliosis and MX dirt bikes in my early 20s. That chart is very helpful thank you! My current right knee weakness could be related to the scoliosis too...but I don't think it would directly be related to the L5 S1 herniation since that was unilateral down my left leg.
But as you said it is all speculation. However I do think this thread is helpful, I've been mis-diagnosed to the point of having a useless surgery and missing one breast. So I am quite jaded when it comes to health and doctors and want to be very prepared before I step foot in a doctor's office. Not that I don't believe in it though, I've had great success with acupuncture and chiropractic. PT not so much and I go through primary care physicians like water. Though both my knee and back surgery increased my quality of life so I am thankful for that.
The knee weakness I decided to make this thread about is a recent occurrence though. Most likely because I just started running buoys and getting serious about my riding. I appreciate the input and welcome more before I figure out what medical direction to go.