The playing surface in the Arena Amazonia is visibly threadbare in places and even groundsman Carlos Botella admitted it was in ‘bad shape’.
There are also concerns over the high temperatures and humidity in the northern city, where England play Italy in their Group D opener on Saturday night.
"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."
Manaus, which lies at the heart of the Amazon rain forest, will stage four matches during the World Cup and because of its high temperatures, drinks breaks could be taken during the games there.
"FIFPro's concerns stretch to parts of Brazil, including Manaus, where heat and humidity can reach dangerous levels at this time of year," the statement continued.
"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.
The governing body has ruled that it will decide on a case-by-case basis whether cooling breaks are necessary at the World Cup.
A FIFA spokesperson said: "The pitch in Manaus has been undergoing treatment in preparation for the FIFA World Cup. Over the last three months, mitigation procedures have been put in place and there continues to be, significant
"Pitch experts from FIFA and the LOC (Local Organising Committee) are satisfied that the pitch will be ready for training and the matches, and have been based in Manaus to ensure that all proper procedures are in place."