I have too formerly male cats - they're brothers and one was the biggest in the litter and the other the runt.
A few days ago they got a scare from a neighbourhood pitbull (yes one of those docile creatures) and the big guy came up lame.
No wounds - I think he just fell off a fence while trying to get away. He's still running, just limping a little, so I don't think that an MRI (which I can get for a cat but not for me) is needed. In a cat-like way, he's laying low, hiding out and waiting to heal.
Where the cat / human observation comes in is how the little guy has been trying to take control - he sniffs weakness in his dominating brother and is exerting control over first to food dish and belly rubs.
Just like humans do.