"The players deserve a quality playing surface and conditions that reflect what is meant to be the world's premier football event. This is simply not the case in Manaus," said FIFPro in a statement.
"Nobody wants to see the players and the spectacle in general suffer."
The grass at the Amazonia arena on Wednesday was threadbare in places and discoloured in others. Near one goalkeeper's area patches of sand could clearly be seen poking through.
Manaus, which lies at the heart of the Amazon rain forest and swelters in high heat and humidity year-round, will stage four matches, starting with England v Italy on Saturday.
Switzerland coach Ottmar Hitzfeld, whose side face Honduras in another match in the city, has already complained about playing "in the middle of the jungle", saying the decision was "almost irresponsible."
"FIFPro's concerns stretch to parts of Brazil, including Manaus, where heat and humidity can reach dangerous levels at this time of year," FIFPro said.
"Putting a player in harm's way is shockingly irresponsible and not how the game ought to be run.
“Cooling breaks are important, but when and how often they’re introduced during a match is also open to interpretation in order to ensure optimal protection for the players”
FIFA guidelines state that cooling breaks are considered when Wet Bulb Globe Temperature (WBGT), which takes into account heat, humidity, wind speed, sun angle and cloud cover, is above 32 Celsius.
FIFA has ruled that it will decide on a case-by-case basis whether cooling breaks are necessary at the World Cup.
However, FIFPro has argued that this is too high, quoting the American College of Sports Medicine as recommending that cooling breaks are introduced when the WBGT reaches 27.9 Celsius.
(Reporting by Brian Homewood, editing by Ed Osmond)
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