Real Madrid midfielder Eduardo Camavinga (21) suffered a knee ligament injury in training with France while club team mate Aurelien Tchouameni (23) went down with a stress fracture to his left foot during their win over Barcelona last month.
Paris St. Germain midfielder Warren Zaire-Emery (17) is out for the rest of the year after he picked up an ankle injury making his debut for the Les Bleus.
"We were right. We knew it. We said it. Repeatedly. And said it again," UNFP said in a statement late on Wednesday.
"But while senior executives are obliged to listen to us, it's rare that they actually listen to us, especially when the interests at stake go beyond the boundaries of the field."
UNFP said it was "high time to undertake a thorough reform of the calendar".
"If the post-World Cup period has vindicated all those who are calling for an overhaul of the fixture list, these are only the first signs of an even deeper crisis that will also end up damaging the spectacle," it added.