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.
"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."
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.
"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."
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.
