Holding midfielder Diarra left the squad on Saturday after missing training for three days because of persistent stomach pains.
The 25-year-old, who has won 27 caps and was regarded as a likely starter, suffers from a genetic blood disorder affecting red blood cells and causing chronic fatigue, his Real Madrid club said in a statement.
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"It's hard to swallow because we know the importance of Lass (Lassana) on and off the pitch," defender Sebastien Squillaci told reporters on another gorgeous day in this scenic French Alps resort surrounded by rugged snow-capped peaks.
"Blows like this often bring the players more closely together," Squillaci added. "We will think about him, he will be in our heads."
"I saw him wake up and get dressed at six in the morning (on Thursday) and tell us he was not feeling well and was going to hospital," defender Gael Clichy told reporters.
"WILL RECOVER"
"He's suffering but he will recover, that's for sure, and this will have no effect on the rest of his career," the France coach told French television channel TF1.
"It's always tough to see a mate go," Clichy said. "We're thinking about him and we hope this will make us stronger. It could have happened to any of us and it's even tougher because he's a good guy. We promised ourselves we would fight for him."
"It's difficult to tell, when you lose a player, if you are going to take another one," the coach said. "We have a few days to make a decision, we will see."
Centre-back Gallas, who suffered a recurrence of a calf muscle injury in late March and has not played since, completed Sunday's two-hour session, raising hopes he would be fit for the World Cup which starts on June 11.
