Speaking completely as a football fan over in the United States and hoping for the sport to become a "top" one over here.
To be truthful I wouldn't want an injury in front of the World Cup to him. For the game I would love us both to be at relatively full strength.
As a completely hypothetical situation if he were to be injured and we (the US) were to win; the mainstream media coverage for what it is on the World Cup over here would say, "Yes they won, but England was without Rooney, blah blah blah." He is a talent and a force. As much as I hate who he plays for in the league I can recognize that on the international level he is a (an ugly) face that raises the recognition of the game over here. He can miss every derby for the rest of my life, that would be amazing, but I would want him in that first game in South Africa.