Liverpool's fixture pile-up is "the latest manifestation of an escalating crisis at the top of the game", says world players' union Fifpro.
The Reds will face Aston Villa in the Carabao Cup quarter-final on 17 December before playing the Club World Cup semi-final in Qatar the next day.
Liverpool boss Jürgen Klopp said on Tuesday "we cannot carry on like this", adding: "We have to find solutions."
Fifpro says many players are concerned about the increasing number of games.
From 23 November to 2 January, Liverpool face a period of 12 games in 37 days in four competitions - the Premier League, Champions League, EFL Cup and Club World Cup - with an FA Cup third-round tie still to be determined in the first week of 2020.
The Reds will field different teams for the EFL Cup and Club World Cup fixtures, which fall within 24 hours of each other on different continents.
Fifpro has canvassed the opinion of players over the past 18 months, including those from the Premier League, La Liga, Serie A, Ligue 1 and Bundesliga.
"A large number of them have expressed their concerns about an increase in the amount of competitions they are playing every season," it added in a statement.
"About half of players who have more than 50 matches per season say they feel stretched by their match schedule.
"We have made a series of recommendations to reverse the trend in our recent 'At The Limit' report and we expect Fifa and other football stakeholders to address these as soon as possible to protect the health of players."
Fifpro represents 65,000 professional footballers from 63 national players' associations. Their 'At the Limit' report compared the workload of 16 players over a 12-month period with medical science on health and performance capacity.
The report claims elite non-European players such as Sadio Mane are travelling too much without adequate rest when they leave club football for international duty.
Mane, who has already played 49 games for club in country this year, featured in 70 matches and travelled 100,000km (62,137 miles) to represent Senegal in the 2018-19 season.
Mane could potentially feature in 12 more games for Liverpool this year as well as two internationals.