That would have created an ugly situation for the MLB...a trainer should ALWAYS put the health of the player above the "product on the field." I think the trainers should have run out onto the field in the middle of the play. The umps aren't medical professionals, and aren't trained to assess the severity of the injury. Can you imagine the controversy if Happ had been hurt, or if there was punitive action taken against a team because the trainer was trying to intervene in a potentially life threatening play?
I agree: play called dead, everyone advances a base, the batter takes 1st, and it's not considered an AB.
Perhaps MLB is already working on this.