Oblique tears sound like a relatively benign injury.......but they aren't. A grade I takes ~4-5 weeks of rest and rehab before returning to play. A grade II which is what I believe he eventually was dealing with can take 2-3 months to properly rehab. He was never given more than 2 1/2 weeks to heal, during which time he was treated with antiinflammatories and injections.
These injuries can occur when there is direct hits to the ribs or when a hit causes the torso to twist/torque..............both mechanisms commonly experienced by a WR. Once significantly injured, significant pain is a universal hallmark............it can hurt to breathe, to run, to turn, to bend and sometimes even to just walk. And any further trauma to the chest or twisting can easily cause extension or recurrence of the injury. No doubt, it can bring the strongest and best conditioned players to their knees.