Speaking from experience, a bad knee at a young age wears out over time. So, he may have had his original speed for a few years after the first injury. But the more cartilage and miniscus damage (and we know there was lots), his knee eventually wore down to nothing.
I was never a professional athlete, but I ripped up my knee (two severed ligaments and accompanied cartilage damage) just before my 17th birthday. I was able to play sports for almost 20 more years, but the last year I was actually able to run was when I was 36.