Uefa president Michel Platini said on Monday that he wanted eight extra teams in the finals from 2018 onwards.
The proposal followed Fifa president Sepp Blatter's suggestion that more African and Asian countries should play at the expense of European ones.
"Never say never, but it's unlikely," said Valcke, number two to Blatter.
Europe has 13 slots in the current set-up, while Africa has five and Asia four plus a play-off place.
Platini's idea for 40 teams would see eight groups of five at the World Cup as opposed to the present structure of eight groups of four.
Russian sports minister Vitaly Mutko said expecting the country to accommodate eight extra teams in 2018 was "unrealistic".
"We've already got to the stage of discussing the calendar," said Mutko. "Everything's possible, but you need to respect the country that's carrying out the preparations for the tournament.
"We're preparing on the basis that 32 teams will be taking part."
Fifa general secretary Jerome Valcke has said it is unlikely that proposals to expand the World Cup from 32 to 40 teams will be implemented.