First of all, what most people refer to colloqually as a “bruised rib” is just a severe bruise to the serratus anteiror or latissimus dorsi (the muscles in front and behind the torso that cover the rib cage) or the intercostal muscles that lift the rib cage. Deep tissue bruising of this area can make breathing or raising ones arms very painful, and because the area is in constant motion it takes longer to heal, and the general treatment is to tightly bind the effected region of the thorax to minimize motion, which also reduces effective lung capacity. This isn’t life threatening unless there are other issues such as internal bleeding, but since the kind of impact that causes this can also cause other internal damage, it is best to be checked by a physician, especially if the patient starts to show urine in the blood, or cannot urinate, or shows any signs of shock.
A genuinely fractured rib, and especially one in which there is a compound fracture (where the rib has separated and cut into muscle or out through the skin) or a flail chest injury (where several ribs are broken inline) is a potentially life threatening injury and will pretty much completely incapacitate the injured party as they will not be able to breath controllably. If the rib punctures a lung the result can be pneumothorax, a pressurization of the pleural space that causes the lung to collapse, which is immediately life threatening, requiring the chest cavity to be depressurized and in severe cases, open chest surgury. Nobody is playing a game of anything except Go or spades with this kind of injury. Although I’ve never had a fully fractured rib I did have a non-pentrated concentrated blunt force injury to the mid-thorax (pectoral region) which resulted in extensive surface bruising and deep contusion (leaving scar tissue in the muscle) with possible minor fracture of the rib, and although it did not incapacitate me in the immedite term it was painful to breathe for the next several weeks; the injury felt as if someone was sticking a red-hot poker into my pec and twisting it around, and I could barely raise my left more than mid-chest level. I cannot imagine anyone with a similar or worse injury could possibly be competitive in an athletic environment.
Fortunately, it is actually pretty difficult for a person using normal body strength to break the upper ribs of an adult because they are so well supported that you pretty much have to break several at once. Even with a concentrated load this is difficult; the adult human chest can support several hundred pounds of load (or equivilent shock), and the kind of force that would be required to break the rib cage would also throw a person back a couple of feet at least. Dwayne Johnson would pretty much have to plow into you at full speed or jump up and down on you while you are lying on the ground to get this kind of injury from a body blow, so most injuries of this nature result from car accidents or falls from height, or else due to impaling injuries (e.g. gunshot wounds). However, the lower, so-called ‘false ribs’ and the floating ribs are not as robust, and especially not in children or even adolescents, and are far more prone to injury from single strikes as they are below the serratus anterior muscles and are weakly reinforced. These aren’t typically as life threatening or prevent effective breathing unless they are driven inward and puncture an organ, but may be even more difficult to heal due to the flexibility of the torso. I suppose it is possible that someone with a fractured floating rib might still be able to compete (never having had this happen I can’t speak from experience) but it seems unwise to say the least.