"We are a little concerned about Manaus - the delivery date will be difficult to meet," Sports24 quoted Valcke, as saying in Sao Paulo.
"For projects which could be late there is no plan B - taking away a stadium (off the list of venues) is still possible. The final draw (in December of next year) will serve as the deadline. That's when the decision will be taken," he added.
"But if they can prove ahead of the draw that everything will be ready by January 20, 2014, that will be good (however) delivering a stadium six weeks before (the World Cup kicks off) in April 2014, no, this is not possible," he added.
Valcke added it was better to sound a warning with a year to go to avoid having the host nation asking thereafter "but why didn't you tell us?"
"When you visit the Maracana site in Rio you ask yourself - 'but how are they going to do it?' But I am not an expert and the experts tell me that everything will be ready for a friendly on June 2, 2013 against England. So we believe them, we at FIFA  believe them," said Valcke. (ANI )