"Messi, forgive them for they know not what they do."

This homemade banner greeted the world's best player, Lionel Messi, as he strode onto the pitch for Saturday's Copa America quarterfinal between heavily favored Argentina and underdog Uruguay. Whether or not Messi spotted the message, he would have appreciated it. For a man who has been dominant for his club team, Barcelona, there's a growing sense that he has failed to deliver for his national team. And that has led to some rather unpleasant criticism from some quarters—including...