With all the usual caveats, my own instinct is to be guided by appetite and energy, on which basis it sounds as if Mosi may have thwacked his paw only?
When Blip had a limp ages ago, it turned out she had a swollen gland, presumably caused by an infected injury on her leg, but she was listless and in obvious pain then.
I hope the little fella recovers speedily, whatever the cause. He is in the wars at the moment, isnt he?