I don't know enough about the symptomology of the spleen problem to say whether it sounds outright negligent of the training staff to have missed that call, but I certainly understand Clark's position.
Spleen injury is extremely tough to diagnose, and it rarely shows up when it initially happens. It's an internal injury, not like a broken leg or a sprained ankle. And I seriously doubt if our trainers or any other ones are equipped to deal with an initial diagnosis.
When I was coaching high school, we had a kid who dropped a pass after taking a pretty hard hit, at a critical point in the game. The other coaches jumped him pretty good. He was laid up on the sidelines, and no one could seem to find out what was wrong. One coach said, "He's got his pride hurt, that's all."
Three days later they removed his spleen and he was done for the year. Now, the Stillers training staff should have much more experience than a HS trainer and a couple of EMTs--but still, I think it's an indication that this injury is not obvious.