Gallas, who has been sidelined since late March because of a recurrence of a left calf muscle injury, is a doubtful starter for the June 11-July 11 finals in South Africa, which means coach Raymond Domenech has to work on contingency plans.
"The big question mark is William," said Domenech, who named a provisional 24-man list instead of the official 23-strong squad because of the Gallas problem.
France have always relied on watertight back fours, notably during their 1998 World Cup-winning campaign and their run to the 2006 final.
They have struggled in that area over recent years, however, partly because Domenech has never been able to find a totally dependable pair of central defenders.
Gallas and Eric Abidal are now his favourite option but if the former Arsenal skipper is ruled out, the coach will need to improvise.
One solution would be to field holding midfielder Jeremy Toulalan as centre back alongside Abidal. The workaholic, reliable Toulalan has been convincing in that position in several matches with his Olympique Lyon club this season.
"It's a solution, obviously," Domenech told reporters at France's training camp in the French Alps resort of Tignes. "He (Toulalan) has showed potential (as centre-back). Then it would be a game of musical chairs."
UNUSUAL DEFENCE
Domenech did not elaborate, leaving French media to speculate on who could take over Toulalan's position as holding midfielder. One possibility would be to ask playmaker Yoann Gourcuff to play a more defensive role.
"We have options, possibilities, things we have seen during the season that would be interesting to try out," Domenech said.
Left back Patrice Evra said France should start to work on alternative solutions without waiting to find out whether Gallas would be fit.
"We have to work on our defence now, with the defenders who are available," the Manchester United player told reporters. "If ever we have to field an unusual defence, we will have tested it and we'll be ready."
France face Mexico, Uruguay and hosts South Africa in Group A at the finals.
Domenech, who has faced criticism ever since his side's Euro 2008 flop and will be replaced by former France defender Laurent Blanc after the World Cup, did not sound at all worried.
"Once it starts, we'll put the chairs into place and start the music," he said.
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