Knee injury that would end in honourable discharge from military but not law enforcement

Hi guys,

I have a character who needs an injury that would see him discharged from the US marines, but still allow him to go into law enforcement after he's recovered.

I've searched USMC discharge+"knee injury" , and "USMC medical discharge procedure" as well as "federal law enforcement medical discharge/medical qualification" and general searches for the types of knee injuries and while informative, haven't really answered my question. I've also checked the injury:broken bones and military tags.

It doesn't even have to be a knee injury. Just something that would see him unable to serve in the military, but still let him join law enforcement.

Any help or advice would be great.