The former England manager insists Rooney is the one player in Fabio Capello's squad who is indispensable ahead of the finals in South Africa.
The Manchester United star was hampered by a groin injury at the end of the Premier League campaign.
Keegan told Bloomberg Television: "England could lose any of the other players and, whilst it would be a blow, it wouldn't be a fatal blow.
"But, if England lost Rooney, I would say they have no chance then because he's the one player who can sprinkle some stardust on the team.
"Everyone will have their key player. Ours is Rooney and, if he performs well and plays to his very best, then I think England can get to the semi-finals."
Keegan, who managed England at Euro 2000, is full of praise for Capello but knows from experience that he will be under pressure if skipper Rio Ferdinand and his players fail to live up to their potential.
He said: "You can be the nicest guy in the world, you can be the best coach in the world but if you don't get results you are not going to keep your job.
"Capello has had a very, very good startbut if they (England) have a poor World Cup, then the pressure will come straight back."
Keegan is tipping Argentina as dark horses to win the tournament.
He said: "They've under-performed under Diego Maradona so far. But they have the players and, if they can get it together for six, seven matches, they have the players to win a World Cup."