Yes, hernias happen all over the abdomen and groin; but in general not in the rib-cage or the lungs. That was what I thought in the first place. He was said to have broken ribs and a bruised lung after being manhandled and knocked to the ground.
Broken ribs are definitely pretty easy to detect; cracked ribs, even. Not that there seems to be all that much to be done about them because they're not easily splinted, but you can know what's happened and retire to bed-rest sitting as upright as possible, and careful breathing, and they ought to mend in between a fortnight and six weeks. (Flail chest is something else again, but I don't think it's suggested that ailed Rand Paul.) Oh, and ice-packs can be really helpful, as are over the counter pain-killers.
It just does seem a little strange that the injuries he suffered in 2017 caused a hernia needing treatment in 2019 but not before then.