Arsenal boss Arsene Wenger says Philippe Senderos will be out for up to three months after dislocating his shoulder at the World Cup.
Senderos suffered the injury playing in the World Cup
With Sol Campbell having left and other defenders injured, Wenger said they may be forced into the transfer market.
"We have lost Senderos for the next three months and Johan Djourou and Gael Clichy have had injuries," he said.
"If we find a player who can play centre back, left back then we are still open."
He picked out highly-rated West Brom defender Curtis Davis for particular praise.
"Curtis Davies I like. I have not made an inquiry or a bid but I like the player."
Arsenal's World Cup players are due to return to training later this week, and Wenger admitted another one of them - Freddie Ljungberg - was also injured.
"Freddie has had an inflammation in his foot," said Wenger.
"We have to check it with a scan. We will see."
It has also been announced that Spanish midfielder Cesc Fabregas will wear the number four shirt this season, which used to belong to Patrick Vieira and was unused in 2005/06 following the Frenchman's move to Juventus.